made a battery adapter today.

I did similar recently by replacing the 14.4V 12 NiCd cells in my Bosch drill with 4 lithium ion 18650s. I used tabbed cells and mounted them to a PCB and mounted a supervisory board above, the assembly is fixed in the battery pack with 3M double sided foam tape. The original charger I gutted the internals just leaving the bare PCB and contacts and routed an external lithium ion charger in through where the mains cable went in and connected it to the contacts. All works great and has given a new

I have a couple of older Bosch tools - a drill and a reciprocating saw. These run on 18V NiCd or Nimh battery packs. My newer tools are Porter Cable 20V LiOn. Rather than buy a couple of new packs for the Bosch tools, I opened up and emptied one of the Bosch packs and added a 3D printed holder for a Porter Cable pack.
Woo Hoo! It works a treat. In coming days, I will refine the design - this was more of a proof of concept deal (and I had some work for the recip saw). There's much room for improvement - mostly getting rid of all the empty space where the NiCd cells used to be, but for now, I'm happy.
============================
Polymer clay like Fimo and Sculpey are PVC resin that can be molded to shape and then baked hard. I don't know how they compare to 3D printed resin for structural strength, but I've found them useful to capture the shape of irregular cavities. They might combine with 3D printing to fit contours you can't measure.

That's really cool. I wish I had the time these days for that sort of thing. I've got a couple Milwaukee M18s drills that have served me well as daily use tools, but I'm down to one good battery. The wife is getting tired of my swiping her Makita drill and impact driver when I need to do something. LOL. She says she bought Makitas for the house because she knew I couldn't steal her batteries.

I have a couple of older Bosch tools - a drill and a reciprocating saw. These run on 18V NiCd or Nimh battery packs. My newer tools are Porter Cable 20V LiOn. Rather than buy a couple of new packs for the Bosch tools, I opened up and emptied one of the Bosch packs and added a 3D printed holder for a Porter Cable pack.
Woo Hoo! It works a treat. In coming days, I will refine the design - this was more of a proof of concept deal (and I had some work for the recip saw). There's much room for improvement - mostly getting rid of all the empty space where the NiCd cells used to be, but for now, I'm happy.
======================================
How do you like 3D printing, compared making custom parts with machine tools or cutting and gluing?
I could see it for prototyping plastic parts that would be injection molded in production but for home projects I usually need more strength and don't care much about cosmetics.
As an example of an irregular shape that isn't too hard to carve out the old way, how difficult would it be to create a CAD model to print the hull of a model clipper ship?

Don't know how hard it would be but there are several models to 3D print here you can look over:

I've found my 3D printer quite useful, the only non practical thing I've printed was the 2 little owls model that shipped with it on the SD card. I've printed soft jaws for holding tubing, a guide to keep the tubing square on to the linisher belt or at 45, a special funnel to aid filling a mould, a jig to place castor mounting plates in the correct position before clamping and welding, and a few other things including a pitot tube holder for one of my neighbours model plane. It did finally force me to get a grip on 3D modelling and I'm using Freecad currently.

Parts with accurately-placed curved tunnels through them (eg for cable routes) are easy with 3D printing; with most machine tools, not so much. That depends on a bunch of factors, including the level of detail that you aim for; what design software you use and your skills with it; and what you start with, such as photos, measured drawings, or an existing model. For just a hull-shaped object, draw a few Bezier curves and surfaces and be done in five minutes, with top-end software; or maybe never with low-end. The thousands of ship and boat 3D print files (some free, some expensive) on Thingiverse and other 3D model sites have been drawn with dozens of different packages, with people spending a few minutes to a few years making and refining the designs.
An accurate and super-detailed model could take weeks or months to draw up, regardless of what software you use, unless you start with an existing physical prototype, scan it for a few minutes with a good 3D scanner, and then spend a few hours cleaning up the scan.

Parts with accurately-placed curved tunnels through them (eg for cable routes) are easy with 3D printing; with most machine tools, not so much.
That depends on a bunch of factors, including the level of detail that you aim for; what design software you use and your skills with it; and what you start with, such as photos, measured drawings, or an existing model. For just a hull-shaped object, draw a few Bezier curves and surfaces and be done in five minutes, with top-end software; or maybe never with low-end. The thousands of ship and boat 3D print files (some free, some expensive) on Thingiverse and other 3D model sites have been drawn with dozens of different packages, with people spending a few minutes to a few years making and refining the designs.
An accurate and super-detailed model could take weeks or months to draw up, regardless of what software you use, unless you start with an existing physical prototype, scan it for a few minutes with a good 3D scanner, and then spend a few hours cleaning up the scan. jiw
=================================
Thanks. I drew the fairly complex RF-tight enclosures for digital radio boards with a traditional 2D CAD program, actually the line drawing feature of the circuit board design program. I had taken a well-taught course in mechanical drawing in junior high school and worked in the drafting department during my apprenticeship in machine design.

The mechanical drawing course, and the drafting experience, both are pretty helpful when using a 3D CAD program. Of course most 3D CADs let you drag the viewpoint around to easily view an object from any angle, but there's still the mental step of imagining a 3D object from the 2D picture on the screen. There are free (limited) versions of some programs, like Meshmixer from Autodesk, and SketchUp, and also some good programs like Blender are completely free. I've seen it compared to Rhino and SolidWorks. FreeCAD and OpenSCAD both are free too. I've used OpenSCAD for a lot of simple designs that I've printed, since it is easy to parameterize a design and is more of a programming language (which I see as a plus) than other CAD programs. However, it would be really laborious drawing a boat with it.

The mechanical drawing course, and the drafting experience, both are pretty helpful when using a 3D CAD program. Of course most 3D CADs let you drag the viewpoint around to easily view an object from any angle, but there's still the mental step of imagining a 3D object from the 2D picture on the screen. --------------------- That's part of drafting and machinist training. Some of the samples we had to draw were quite challenging, like the spatial relations section of an IQ test. The teacher had a trick of making coins stick to the wall as though they were magnetic to wood. He used his knife to peel up tiny sharp prongs around the edges. --------------------- There are free (limited) versions of some programs, like Meshmixer from Autodesk, and SketchUp, and also some good programs like Blender are completely free. I've seen it compared to Rhino and SolidWorks. FreeCAD and OpenSCAD both are free too. I've used OpenSCAD for a lot of simple designs that I've printed, since it is easy to parameterize a design and is more of a programming language (which I see as a plus) than other CAD programs. However, it would be really laborious drawing a boat with it. jiw
=================================================== Thanks. Before buying a 3D printer I want to confirm that I can create my own 3D models, with a program less complex and expensive than the SolidWorks that Segway used. I gave a hull as an example because I'm familiar with the lofting process, having designed and built such models as a kid.
I adjusted the sails to make the model boat travel at a right angle to the wind, back and forth along a straight line as low level turbulence flipped it around, and usually return near the launch site.

the 2D picture on the screen. ... jiw

A couple other free CAD systems to take look at, would be Fusion 360 and Onshape.
Fusion 360 is made by Autodesk. It has CAM capabilities, although the free version has recently become somewhat limited. The built-in slicer is not free, but it will export STL files so you can use your favorite slicer. Runs on Apple and Windows.
Onshape is running entirely in the web browser, so it will work on any computer platform, even your phone. No built-in CAM capabilities at all.

[snip] I have a nice Milwaukee cordless hammer drill that ran on 18V NiCad packs. When these started to die and became unobtanium, I started to look around. Grizly Industrial (on-line and in bellingham WA) had (still has?) a nice set of Lithium Ion plug-compatible 18 Volt packs that fit the drill together with a 'smart' charger.