I know ways to build a coaming that would work from my pattern making experience but they are too time consuming and not practical and would be costly. You guys might play around with the slowed reinforced epoxy that might work. I never tried anything like that. As it was wiped you could sprinkle 1/4" strans of fiberglass for strength between the wipes and press them in with your gloved finger so on your second wipe they wouldn't come to the surface and spoil you surface. If anyone tries epoxy you have to prepare the wood surface of the mold well and don't use crisco as you would for body putty. I would shellac it, then sand to smooth the surface and then shellac again and again until shiny. Then three coat of paste wax, each shined and then a spray parting wax. There are ways of making a good hatch seal but again they are not practical and would be time consuming and costly and something can always go wrong.

As for plasters, I have a lot of experience in molding plaster wiping and cutting. I have built most of the models of plaster then transferred to epoxy masters for decorated bottles you see in liquer stores and grocery market.

John