it might help if you provided a sketch of photo of your big plan.
I've never enjoyed working with foamcore. Then again I never had a really sharp knife either.
I'd suggest using blocks of the pink EPS foam that you can get at home depot/lowes spray mounted together to get all your crazy shapes, and then cover it in latex paint before fiberglassing.
I think you can get better and more stable form from building blocks and masses rather than trying to surface model everything.