Mixed emotions on one that is heat treated. If you get careless with a hard one, you can do minor damage to the major diameter of a tap, so it might be better to sacrifice the hole size from several repetitions of use, slowly enlarging the hole as you turn your tap. Unless you're involved in some serious tapping, using it daily, I dare say that will never be an issue.
Hole size? The smartest thing you can do is mic the taps you'd like to use and size the holes accordingly. The major diameter of taps is greater than nominal, so you'll find them slightly oversized. Measure the body, or shank of the tap. Make your hole such that the guide can be slipped off with the tap still in the hole if necessary. You may find you need a bastard size, but the closest size you have at your disposal will work, providing a little extra clearance, or you could bore the holes if you're not in a hurry. With the number series, you shouldn't have much trouble selecting a drill that will provide the proper clearance, but larger taps can be a problem. If you drill only, double drill to insure hole size and roundness.
Taps in the number series, below a #6, have a body diameter greater than the major thread diameter, so the tapping block won't be able to be lifted off without removing the tap from the hole. You might decide to make the pilot size the body size in that instance.
================== Two types of the type of tap guide you are describing.
First type guides off the threaded portion of the tap. Easy to make and use. I have seen these both flat and with a V for tapping cross holes in a shaft. Most likely better off if not hardened.
Second type guides off the shank of the tap above the threaded portion. The tap shanks are a standard size and this is most likely a better type if you do lots of small holes as the shank is bigger in the small taps [ i.e. < #10] You can harden but no real need.
a whole bunch more google for "tap shank dimensions" or "tap shank diameters"
The commercial units are finely machined and the holes laid out in a artistic spiral pattern but from a utility standpoint a simple block with a straight hole a few thousandths over will get the job done.
This was one of the first projects in my machining classes. It was always a revilation to the younger students that you could make rather than buy tools.
I'm almost positive that the Workshop Series - Making Small Workshop Tools has all the info you need on making tapping guides.
I made some a few years and have used them without any dramas ever since, they are made of some mild steel rod, squared off in the lathe, with a hole about 1/64 larger than the tap nominal size drilled through the centre. Length varies but mine long enough so that all the thread on the tap is inside the guide before I start. Diameter also varies, seems like I used about 3 times the hole diameter to the diameter of the guide with a nice little "step" on the bottom of about 5 times the hole diameter.
The photo doesn't enlarge so you can't see a lot of the details. Has several different bushings that you can swap in, the bottom of the holder has a V-groove in it, very handy for tapping rounded surfaces. Would be easy enough to make if you've got a mill or a shaper.
Was a lot cheaper when I bought it, but if you value your time, it's still a bargain.