Core Suggestion for Moldless(?) Hull

I'm a senior mechanical engineering student building a double-hull solar-powered boat for a design competition. I've been doing a lot of reading and I am leaning toward a moldless layup using kevlar and fiberglass and im curious about what the best core material would be. I've read that polyurethane foam is the way to go, but I am wondering if regular rigid pink insulation foam will be good enough. I've read its more friable and not quite as good to work with, but can we make it work?

Also, is there a strong reason not to be doing a moldless construction? It seems to me that since we're planning on having foam-encased fiberglass hull, we'll be ok making the moldless hulls. I realize ill be sacrificing consistency between the two hulls and we'll have to do more to finish the surface, but are these reasons to change my plan?

I'd reall appreciate any insight. Thanks.