Well, y'see, I live in Australia, and it does get very hot over here, and from what I've just heard from a friend of mine, the material that the DP Helmets are made from is a soft vinyl, and it tends to warp in heat. So before I heard that, I figured that vaccum forming it out of solid plastic might be the way to go. Now I'm not so sure. Remember I am new to costuming, and I am getting confused.
Make a silicone or latex mold of the helmet first. Then build up a good layer of plaster on it. Then pull the mold halves off and let dry. Cast your helmet in your new mold using plaster. Now de-mold after about two days drying time. You have a plaster positive. Now heat your styrene in an oven till its droopy. Put it in your vacuum molder. Oh did I mention it was difficult? Mold your helmet using the vacuum pressure. There you are done!
I think I also forgot to mention the drying time between steps. Oh and did you make it a two piece vacu-mold? Watchout for those undercuts!