The easiest way to make a round hole in a plexiglass bubble canopy is to use a remington grit edge hole saw , remove the drill bit pilot and substitute a piece of plain round bar the same diameter.
Do your hole sawing on a warm day, the plexi will be more flexible and less likely to crack.
Start by very carefully putting your pilot hole where you want the vent hole. Use masking tape as needed to prevent slippage and accidental scratches
Make sure the pilot bar is an easy sliding fit in the hole. Applying very light pressure and a slow speed, cut half way through the canopy, now go inside and cut almost half way the other way. Then go back to your original side where you started and gently cut unti the center disc starts to rotate, stop and remove the cutter at once. keep an eye on the cut as you go, to make sure you are cutting evenly , you do not want to cut all the way through on one side and barely touch the other
The idea of cutting from both sides is to avoid a thick and thin spot on your cutting and to keep the break through in the center of the plexi. Now trim up the ridge in the middle, and polish the edge with a dremel tool or similar high speed grinder, and you are done.
EAA chapter 286