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.
Step 6: Molding!
Take what ever object you want to mold and place it in the center of the pegboard box. Turn your oven to broil and let it heat up. While your waiting for it to heat up move your pegboard box as close as you can to the oven being quick is key the plastic cools fast so the quicker the better. Also make sure your shop vac is sucking not pushing. Once the oven is hot place your 2x4 frame with the plastic into the oven make sure to wear oven mitts as it gets hot. At first the plastic will start to bubble and get wavy and then settle back flat this is when it is time. Take out the 2x2 and place it over the mold turn on the shop vac and hold the frame for a minute. Then your done! Just be careful to not break the plastic when digging the mold out. ( I'm sorry I don't have a picture of the shop vac hooked up but I'm sure you can imagine what it looks like!)
Step 7: Things to Mold
I have made a Light Saber, and Iron Man Mask (Which I will soon make a tutorial for)
Step 8: Things to Note
Say you molded something but it didn't come out just right and you want to try again. Wait for the plastic to cool then put it back in the oven and it will reform itself again. You get about 3 or 4 uses of the plastic until it gets thin and breaks easy. Good Luck!