Regarding modifying the shape of a helmet: if you're using heat with help of, say, a heat gun, then heating the helmet and then rapidly cooling under cold running water would do the trick. I've had to do some modifications to a Rubie's plastic helmet, and this method not only works, it holds it shape well. The trick is to heat slowly and the area around the immediate area you want "retrained" so that the "memory" of its original shape does not assert itself in the modification.