To bring this back.....I sanded things down and discovered a piece of metal that was poking through the plywood. Kind of looked like a large staple? Not sure if this moved because of heat or what, but it looks like what caused the fiberglass to crack.
I ground everything down as best and neatly as I could. My neighbor suggested I use his Epoxy Resin. Two steps. First, mix it up and layer in some cloth. Let that sit....then mix more with a thickener. Put that on top. Then I can sand it smooth, paint or cover for cosmetic needs. Hope it works.