Here the skull has been glued down to a board and I've put plasticene in the eye and nasal cavities, plus all around underneath where the rubber might flow and lock onto the skull.
This is the skull with a paper cup around it, hot glued to the board, in order to keep the silicone rubber from leaking out. The rubber is a viscous blue liquid that is starting to fill the cup.
My plan is to cast this mold in resin, then add hardware to it and modify it so that it looks steampunk. As I mentioned a couple of posts ago, how I accomplish that is a bit vague at this point.