check out my thread "Working with Foam" That should help you get whatever shape you want. As for foam choices, i would not recommend spray foam. Just doesn't hold it's shape properly. Use a rigid foam like blue or pink EPS as found at Home Depot, if not an actual prototyping urethane foam.
You may want to consider making a female mold out of fiberglass and then pulling a male part from it. That can save you alot of time finishing and will allow you to pull multiple parts easily should you decide to try something different.
If not, just make sure you use a good mold release when you're applying your fiberglass, and that part should come out nice and clean after you do the sanding thing.