Materials for molding and casting

[xXDashIVXx]

So...
Christmas is coming up and I think the only thing I am going to get is the stuff I need to cast and mold. I am thinking some smooth cast onyx fast and smooth on mold max 30, plus some mold release. But I dont know what to do for the jacket. Should i make it out of fiberglass or something else. Besicly I want to know what you recomend/prefer for the process, and if I left anything out... thank you!

 

[xXDashIVXx]

Also, do I need some clay to make the jacket? Because my jacket is going to be on the timmy helmet, should it be a matrix mold, or do you think I can get by with just two halves. I plan on slush casting