Okay I've read everything I could find and experimented a bit and I am ready to drill plexiglass. Well not quite. As usual, instructions and tips are all over the spectrum.
Perhaps the most daunting words were Tony Bingelis' warning, "In my estimation countersunk screws should not be used...." (page 214, SCT). Well that's great. Only 100 or so on the tipup.
I also looked into putting a washer between the screw head and the plexiglass in the thinking that it might distribute any stresses, but it requires more countersinking than for just the screw and maybe weakens the plexiglass too much.
No consensus on how much relief the hole has to have on the body of the screw for expansion concerns; the new plans say 5/32 but older posts say 3/16 and the aluminum to #23.
I wouldn't obsess about it but the post that stated that 35%, IIRC, of respondents had cracks develop in their canopy, either during fabrication or thereafter, is worth one's attention.
For those interested, here is a close up of the hardware I rounded up.
Top row is 6's, bottom row 8s. From left, AN507 screw, respective tinneman washer, NAS finishing washer, mockup of screw seated, hi-dollar zero flute countersink and 110 degree 3-flute countersink 3/8"dia. (edit: the NAS finishing washer is second, tinneman third. DA).
As with every other non-piloted c-sink I've ever used, the zero flute consistently wandered off center when used freehand (by me), especially when trying to countersink before "drilling up".
The 110 degree c-sink leaves a much better fit for aluminum side skirts, but will not be wide enough for washers. It takes a .5 inch dia. size. Also I am assuming what you want is a "sloppy" fit, so the plexiglass can expand and contract a little everywhere, not just in the area of the screw body.
Finally, I got out a scrap of canopy, beat it on the workbench until my hand hurt and then bent it 200 degrees without it breaking. Took another piece and carefully drilled a 1/8" plexiglass hole. Halfway that is. Snapped like cheap window glass.
So much for the reassurance of plexiglass bits. The kicker here is that if you look closely, one side of the scrap of plexiglass was not polished. Also, the temp in the hangar was 68F. The crack appeared to originate at the bottom of the drill hole but could have come from the stress being transferred to the sharp edge.
So far all I am sure of is that plexiglass must be heated till it softens enough to work on it and everything gets polished. I have kept heaters on the canopy at all times during the fitting to keep its temp 80F or above before moving, cutting or polishing it.