The best way to get a big hole in acrylic, like for a bulkhead, is to use your hole saw to make a "template", just cut a hole in a piece of wood. Then use that to rout the hole, using a flushtrim bit. You attach the wood to the acrylic with double sided tape, drill a small pilot hole for your bit, then rout out the inside of the hole. Make sense? If you don't have any of this stuff, you can probably make a passable hole using a hole saw and drill press, set on the slowest speed, or a handheld variable spped drill. Just go slow and try not to heat up the acrylic. Cracking isn't as much of a problem as melting is, at least with extruded acrylic. The routing method produces a much cleaner hole, but if it's only for a bulkhead, you don't need it that clean.