I prefer Silicones,rather than urethane rubber,or latex. The very best is Dragonskin,from smooth-on. Dragon Skin silicones are high performance platinum cure silicone rubbers that can be mixed 1A:1B by weight or volume and cure at room temperature with negligible shrinkage. Cured Dragon Skin is very strong and very “stretchy”. It will stretch many times its original size without tearing and will rebound to its original form without distortion. Also you can do a casting without an undercut in the back of the head,it dont need it,because its that stretchy. It is expensive,but i say it worth it,the casting is never rot,never shrink,tear with age and its easy to patch with its own material.