I'm taking it that you have molded a helmet in a rubber material and are looking for a resin to cast it in?
If so bondo is not good, you probably could but would take a lot of work. You're better off using the polyurethane resin and fiberglass cloth that WalMart also sells. Cutting the cloth into 3" X 8" strips , dipping in resin then laying up in mold. Use more of the accelerator in the mix so it sets up faster. Good luck