To do vacuum forming, you need to:

• stretch a piece of plastic sheet over the mould
• suck the air from the mould
• heat the plastic until it softens enough to bend into the shape of the mould.

To achieve this, I needed to arrange a vacuum chamber with a reasonable seal between the plastic sheet and the mould.

I drilled a large number of small holes through the deepest part of the mould, then flipped it over and built up a thin frame of stand-offs to make a "plenum" behind the mould.

I then glued a sheet of 1/2" plywood across the back to finish the plenum chamber. A hole in the back of this later will allow a vacuum pump to evacuate the mould.

The plastic sheet needs to be held in place and a reasonable seal needs to be made between it and the mould. So I put caulking around the rim and smeared a layer around the edge of the mould.

The hinged frame closes with catches and holds the plastic in place.
 
 
 

I built a stand for the mould, with a dowel pivot on the mould so it can be easily rotated. The mould is quite heavy and I didn't want to be heaving it up and down countless times.

Here it is with one of my many early failures (this time due to not having an adequate heat source).

I later added a couple of handles to the sides of the stand to make it easier to carry around.

I drilled a 2 1/4" hole in a piece of 2x4 to take the hose of the shop vacuum. It's not a very tight fit, but the suction holds it in place well enough.

I drilled a smaller hold in the back of the plenum so the vacuum hose doesn't get pulled in and block the plenum.

Here's the shop vacuum hose in place.

At this point it's ready to go. You need a powerful heat source that can be applied quickly and then removed just as quickly, as the degree of softening is quite critical: too little and it won't get into the corners, too much and the plastic will melt completely: a hole forms and you lose suction.