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I really like your method of creating a seal around each half of the model with play doh. It's so much easier and quicker than trying to cut the shape of the model into a base board and sealing the gaps around the edges after. For small models like that shell though, I would suggest using mold putty to create a mold. It's significantly quicker and easier than using plaster which has a tendency to break when de-molding, espiecally from objects with fine detail like that shell. Plus wax is delicate to begin with so fine detail won't survive a rigid mold being ripped off.Also worth noting with wax is that it will crack with sudden temperature changes. In other words, don't put it in the freezer to dry quicker or it will fall apart when you take it out.