Creating a part inside another one

Hello

I want to create a very simple model of a human head inside a helmet exactly like the picture below ( just the simple shape of the head no eyes mouth , ....)

I have the helmet file attached to the post so if any one of you guys can make the model for me or atleast tell me how I can do it it would be great .

one more thing that I can do which is good as well is to make the helmet solid and scale it a little bit smaller and use it as the head but again I can not do that either. i tried filling all the holes of the helmet but cant make it solid and knitting didnt help too!

thanks alot

SolidworksParts And Features