I used Smooth-On's Rebound 25 to make the mold. The mold itself will have two parts; front and back. I built up a dam with clay to pour the front side. I went ahead and included the wooden framework that will hold the outer hard shell at this point so the mold and frameowrk would all line up.
First comes a thin layer to preserve fine detail (left). It's clear enough to see where bubbles form, so you can pop them. Then come aditional layers of rebound 25 mixed with a thickening agent to build up a thicker mold (right). The bumps (right) are registers which help to align the mold within the exterior hard shell to come.
The hard shell will be applied in three parts; one for the back, and two for the front. This will allow the mold to retain structure around the details around the temples of the helmet. However, the mold itself doesn;t have a seam down the center of the front, which would be unsightly.
The hard shell dries with a pretty nasty/oily surface. After it dried, I sanded it down for easier handling during the slush casting process.
