Need help making a mold and choosing a silicone?

my dads birthday is coming up and he recently bought a mauser. he wants to shoot it a lot but left le doesnt because of the recoil, it left him a huge bruise on his shoulder. i want to make a sleeve to put over the butt of the rifle that can take a lot of the force so it doesnt hurt him anymore. im really good at building things but ive never made a mould or casted anything i really need to know what type of plastic should i use for the sleeve and how would i make the mould for something like this