ORIGINAL: Campy
I am going to try my hand at vacuum forming. I have built a 10" x 12" x 3" box and a 750 watt hot plate in a reflective foil lined box for a heater and I have a 10 gallon shop vac for the suction (bag and filter removed for this).
I'm guessing that your "hot plate" is a single round burner thing. You may need to position a reflector over it to block the direct infrared to the plastic directly above it, and bounce the IR around to the sides and corners. Otherwise you're likely to have a serious hot spot in the middle. (And the ends of your rectangular plastic may not get hot enough before the middle gets too hot.)
That's pretty much what I did for my two-burner-hot-plate-and-disposable-aluminum-pans oven:
http://www.rcgroups.com/forums/showthread.php?t=621858
Also, a shop vac isn't especially appropriate for vacuum forming. (But it will probably work fine for canopies.) It's a myth that shop vacs pull harder than household vacs. (The strongest-pulling vacuum cleaners are canister vacuums with two-stage blowers.) Any high-amp (or high-wattage) vacuum cleaner is probably fine for thin plastic, including a $5 canister vacuum from Goodwill.
(At any rate, the "gallons" aren't relevant. That's just the size of the bucket that the air gets sucked through, not the strength of the motor. Some wimpy shop vacs have big buckets, and some powerful ones have small buckets.)