Here is what I made for small vac-forming projects by
Ralis Kahn. I used a 6.5 hp shop vac on 1/16th thick high impact polystyrene (HIPS) and ABS sheets and it works great for what I needed it for.
Please be very careful with the wiring! Do not attempt to build if you're not familiar with electrical circuits, failure to do the wiring correctly may short the circuit and could cause a fire or even death!