Clamping the plastic
This is the cheap and dirty approach. The wood screws will only last so many uses but can be replaced with flat head machine screws and wing nuts. This plywood is quite smooth. If you experience problems with the plastic pulling away from the frame when the vacuum is applied tiny nails can be used around the 4 sides of the frame so that they dig into the plastic when the frame is clamped. Ironsidz had his friend Ted, Who makes vacuum form packaging, vet this process so "tips from Ted" have been incorporated. Thanks to both.