ORIGINAL: flipstart
I basically wondered how to seal the box so as to get a vacuum pull on the sheeting. Flipstart
1/2" MDF board, 1" drywall screws and silicone caulk. Make a box with the top being about twice as big as the largest dimention of what you are vacuum molding. The top of the box should have lots of small evenly spaced holes drilled in it to let the air out. On a side or bottom, cut a hole for a shop vac hose hole to fit in. It only has to be tight enough to hold the hose in when the vac is turned on. When you assemble the box, predrill all screw holes and get the box fully assembled, mark each peice so you know which part is which and take it all apart. Squirt a thick bead of caulk along the full length of one edge, then screw it together. Repeat for each joint. Save the top and bottom for last. I have seen people make the top removable, but this isn't required for most people and would require additional work to seal since resealing with silicone everytime you opened it would be a bit tedious. Then you make a frame to hold the plastic that is the same size as the top of the box. It should be able to withstand heat since you will be putting it in the oven to heat the plastic.