The Terran Marines from Starcraft are pretty similarly designed to space marines, and I based my Terran Medic costume
off of it. I carved my costume out of blue insulation foam, so if you think my advice might be of any help, feel free to ask.
You seem to have gotten most of the dimensions down pretty well... Did you coat all your stuff in fiberglass? If so, you should be warned that it doesn't stick very well to duct tape.
You can try bondo filling putty to smooth the surface out a little more, but I'm afraid glassing never seems to result in a good, smooth surface unless you can lay it very flat or lay it up against a negative mold of the object. It's the reason I'm probably going to forego the glass on my costume and just seal it with epoxy, even if it's not nearly as strong.