I have had great luck with building up my molds with silica sand and epoxy. It is cheap and easy to use. I mix about a cup of resin and harder, then add enough sand to make it about as thick as mayo. It trowels well around all complex shapes as well. Build up the mold about 1/2 inch thick with this mixture. I have been using this process for many years without any problems of mold shrinking or twisting.