Moulding it would be tricky, but heres a great idea, seal the neckhole, and split the mould vertically. This should give you a non-hollow helmet mould. Then, put a buck in that it will get moulded around, and when it is moulded, open the mould, then pull the buck out of the helmet. It should leave a large hollow area in the helmet. Then remove the area for the visor, rinse, and repeat.