Before casting the plug there's one more thing to do – I need a couple of 'handles' to help pull it out of the mold. The idea here is to embed 2 cap screws with nuts and washers in the plaster. When the mold and plug are apart, the cap screws will be unscrewed, leaving the nut and washer inside the plug.

I made temporary supports out of wood from my scrap box for a couple of 1/4-20 X 2" cap screws to be suspended over the mold. The slotted holes in the cross piece will allow each support to be slipped out from under the bolts when the cast is set (otherwise I'd have to cut them off the bolts – been there, done that!). The threads above the nuts were coated with Vaseline to help them unscrew later.

The picture gives a better idea of what's being done than the description.

Now it's time to cast the canopy plug for vacuum forming.