Yeah the big risk with printed helmets, and casting them in many poly resins too, is warping in heat. So the higher melt temp of the material, the more it'll resist warping in hot weather too (although the actual melt temp may be >200, it'll still soften slightly at half that which can make your helmet warp under it's own weight just sitting on a shelf in sunlight for days on end).
Two approaches are either to print it fairly thick and with a stronger higher-melt material like ABS or PETG, this sorta front-loads the work into much longer print times and possibly more failed prints from the materials being a bit more difficult. Or print it in PLA then fiberglass the inside. Even with this method though don't print it super thin, or just the heat of the resin curing could warp the helmet too.
Fiberglass resin is pretty nasty stuff though, so thicker PETG will probably be best. You'll also be able to design internal details and stuff if you want, that glassing the inside would just cover up.