Richard, I realize this may be to late to help, but a 3 flute drill will deflect a lot less, and short of a gun drill may be the best tool for deep drilling aluminum with carbide of course being better than HSS.
I knew this and actually have a couple 3 flute drills for this purpose. I don't have any excuse for failing to mention it, other than I just didn't think of it. Of course you need a suitable maxhine to start it straight. I expect you could use a hand held drill if you have the skill to start it "straight enough."
-- Bob La Londe CNC Molds N Stuff
--------------------------------------
This Portalign is on UK eBay though the seller is in Michigan.
formatting link
The aluminum base is too flexible to call it perfect for the task, but they are pretty good. For drilling steel they need to be clamped to the work somehow. If the two rods are extended through the base they will center the drill bit on the edge, within the device's flexibility. Drilling a small pilot hole first helps initial alignment considerably. Starting with a "center" drill is even better.
formatting link
I bought a chuck that stays on the male threaded end of the slider shaft instead of borrowing it off the drill.