If you have a Goodwill Blue Hanger store near by, you can usually get a fine vacuum for vacuum forming---just as good a good shop vac, and better than a crummy shop vac---for about $5.
See my "chapter 2" draft. It tells how to pick a vacuum cleaner at a thrift shop, using a piece of craft foam as a vacuum gauge.
http://www.tk560.com/phpBB2/viewforum.php?f=1
It also gives various ways of building plastic-clamping frames that let you use bigger plastic in your oven than the wooden frames from studiocreations.com. Those boards are fine for small stuff, but you really don't want to lose 3 or 4 inches each way when you're trying to maximize the size that will fit in your oven.