Pictures of the real thing, as well as art of more toony subjects, is a great way to work these things out.
I'll take a guess at the material, it appears to be carpet padding. I would be careful, this stuff could be glued or bonded with chemicals that might cause a problem. Your head will heat up while wearing it, out gassing of the foam and it's components might not be good. It is also not the most durable, it may come apart in a short time. There is a fair amount of stress, putting the head on and off. Hopefully it's new, and will last long enough to be worth it. I would not even want to think about using old carpet padding.