This is using basic slush casting with liquid latex to make hollow, flexible shoes for stop motion puppets. I cheat and sculpt one shoe for both left and right - the puppets don't seem to mind. Ordinary casting plaster will do, but I used Hydrostone. Many props can be made by casting in liquid latex. Be sure to use mould release - clay and water works well - on the flange of the mould, where plaster meets plaster, but not on the inside - the plaster needs to absorb the moisture out of the latex so it leaves a thickened skin on the surface.
Thanks to Kevin MacLeod for the music, from his Incompetech site. This piece is Padanaya Blokov.