Mine have a separate vac formed inner shell spaced so the visor can slip up and down.
Helmets are from now disbanded Hi-Impact Props and Costumes Group. (self fabricated)
Here's a few images of the screen used helmet at Celebration 1.
Note it just has foam inside.
Hmm - Just foam and some ear cups - Looks easy enough! Thanks for those pics - It was hard for me to find anything showing the inside of a screen used one clearly!