Here I’ve boxed in the front half, preparatory to pouring the plaster mother mold half. It includes a pour sprue and vents at all the high spots.
It’s made from foam core, hot glue and packing tape. I think this is a way to work faster, and have a smaller plaster mold to deal with. I have usually made these boxes from wood, but this is easier to fit to the shape of the model. I hope it will be strong enough to support the weight of the liquid plaster. The inner surface of the foam core is lined with packing tape, and the whole clay surface will be sprayed with mold release before the plaster goes on.
Vents must release the air from the mold at every high spot, or the poured liquid silicone will not be able to fill the mold entirely. I may still add a second sprue; While the silicone is forgivingly self-leveling, this mold has so much topography that it may be helpful. I can also thin the silicone to help it flow more smoothly.