Ya that would be cool, nice job. Im not sure i fyou can just go straight to fiberglass or not, dont you have to do the rubber and lots of other stuff? I dont know much but ive read a lot. Just wanted to warn you.
You can use whatever clay you're comfortable with, some people think a harder oil based clay is preferable to produce the smooth as glass effect of the helmet but someone who is experienced with water based clay can produce the same effect. If you're a beginner I'd recommend a softer oil based clay. Something like Chavant "le beau touche". I'm using Klean Klay because it's all I had but I wouldn't recommend it for sculpting as it's a bit sticky or something.
I do plan on beefing up the bottom jaw I don't think it comes up high enough.
I'll try to take as many pictures as I can of the process.
Still have to refine some forms and smooth it all out but here is an update.