Bill Schwab writes:

Space is tight, so that has
pushed me toward 4-40, which is a pain to tap. I am thinking of using
one 8-32 screw and making a tight-fitting slot to prevent rotation at
each connection. Any preference or other ideas?

If you get the thread-forming ("cold forming"), instead of cutting, taps
(available from Enco, etc) then miniature aluminum threads are easy. I do
sizes like 0-80 and M1.6 all the time. The only time I've broken a tap is
when I dropped it! Different tap drill sizes are required. No chips and
no back-and-forth, just screw it straight in and out!

In such a situation, how close to the edge of an Al part would you
consider it "safe" to tap for 4-40 and 8-32, respectively?

One or maybe two thread heights. Thread height for 4-40 is 1/40" * sqrt
(3/4) = 0.022". So somewhere in the 20's of thousandths minimum.