Well, the issue is making the mould. I plan to cover whatever I use in fiber glass, so the strength of the material shouldn't matter.
It usually boils down to either cardboard (for flat surfaces) or clay for rounded surfaces. The typical process is make your mould, cast a plaster negative and then fill the negative with glass.
Unfortunately, for something as large as a marine costume using something like traditional clay would be very expensive.
I told my sister that if I pull this off, she would have to do a full prosthesis Kerrigan costume