My stringers and transom were done by a buddy who was the head glass guy at Hustler, plywood has superior adhesion to glass over the foams. He did go on to say that the real strength is in the laminate. When he did my transom he eliminated the KEY cause of rot, this was done by squaring off the bottom of the plywood then building up the plug area with solid glass. He said there was structural no reason what so ever to fill in the V area with wood, that all it did was make it liable to fail again, he also isolated the stringer wood from the transom wood. Reason being the moisture wicks from the transom into the stringers then slowly creeps forward, while you are at it check the stringers more than likely they are wet. Mine were wet up to just past the engine mounts, he cut them out about an other five feet until he got dry dust when cutting. Then installed doubled stringers, added bulkheads and transverses added maybe 100lbs of structural soundness to my 22.
No matter what your beliefs are "GOD BLESS AMERICA"
Fully retired marine tech near 60 years in the biz.