I like the foamboard for the template and fitting to the boat vs plans. I'd use a 1/2" piece of Coosa board (sold at higher end lumber firms or those near the ocean) as the core. Then use either the West System or Total Boat (latter cheaper and just as good IMHO and available on Amazon) along with some stranded fiberglass mat. Maybe 2 layers of the stranded mat and then finish with some fine woven mat (3 total layers). I'd start with two layers one with a 4" overhang followed by the second with a 2" overhang on the edges, then the 3 layers on both sides. Should be more than strong enough. You can brush on the layers of gelcoat after and can put something on the top layer for some grip. I believe they sell it. If you don't want to jump in this hard, try fiberglassing for another small project to see if it is for you. If not, see a local shop. They will do it and it will look professional.
tx