You could always something like fiberglass resin (epoxy resin) or a casting mater like SmoothCast 300 (polyurethane resin). The only thing to keep in mind is that both of those cure by an exothermic chemical reaction, so you neat to watch out for heat. If the material is too thin, the resins could cause warping to the piece, Another Alternative is to just mold the piece as is, and cast copies of them (assuming that it is at least strong enough to retain it's shape with the silicone on it).