This Instructables will show you how to make a very cool Vacuum Former box for just about $30!
Step 1: Materials
Step 2: Make the Frame
To start out make a square frame out of the 2x4, mine was 22"x 24"
Step 3: Make Vacuum Former Plate
Now take your 1/2" Plywood and cut out a square 24"x26" next cut two 3"x 24" pieces and two 3"x 26 pieces. Take the 2x2 and cut 4 2.5" pieces. take one of the side pieces and drill a large hole into it mine was an inch and a half but could have been larger. This is where the shop vac hose will connect to. Then cut out piece of pegboard the same size as your base, or 24"x26".
Step 4: Assembly
Take the 4 2x2's and screw them from the bottom of the 24"x26" base then take the four 1/2" sides and screw them on.Take the pegboard and screw it onto the 2x2s. There now you have all the pieces required.Then take the shop vac hose and push it into the hole and secure it. I used duct tape but am currently working on a better solution.
Step 5: Plastic
I ordered mine from http://www.usplastic.com/catalog/item.aspx?sku=43330&catid=715 the actual plastic is cheap but shipping is an arm and a leg. If you can find polystyrene anywhere else .02 to .04 inches it will do just fine. Take the plastic and cut out a square that is the size of the 2x2 frame then staple it on nice and tight.