Well when I made my helmet I didn't completely cover the top fin part of the helmet. So this way you have a hole in the top that you can pore the stuff in. To make it hollow you have to put the molding stuff in then carefully put the helmet in so the liquid goes in it. An easier way is to put a head in the helmet and then put it on. Talk to me if this is confusing. Lol it probably is.

try to take pictures using zoom