a few corrections to make here from my experience in reinforcing.
Fiberglass Resin is what will eat away styrene and plastic slowly.
Casting resin is the solution. It will not eat away plastic.
So what I recommend for helmets is prewetting your fiberglass cloth with resin and laying it down very carefully.
If you have never done either glassing or resin casting I WOULD NOT recommend doing this since it can turn into a nightmare in that case !
gm