just about anything would suffice as far as scuplting and i use plaster strips from the local hobby shop - get them for about $10 a roll and then back them with fiberglass - once thats done just pour in your latex, and make sure the entire surface of the insde is coated with latex. once the latex cures you can pull it out - if the cast is smooth enough you wont NEED a release agent, but it couldnt hurt
making the cast you might wanna try a release agent - PAM is the best!
if you are doing the green hornet mask, why not just make a silicone mold and cast it in resin? but since this is your first go around a plaster mold and a latex cast might be easier.