The transom is my first big project. My plan was to build a template out of lauan, raise it from 20" to 25", and enclose more of the opening. My lamination schedule is going to be a layer of heavy mat on the outer transom skin, to seal the 43 holes in it, a layer of 1708 up against the inside of the transom core for strength. The core is two pieces of 3/4 Okume marine plywood. The resin I am using is vinylester from US Composites.
Core test fit
Core and 1708 in place with a pair of strongbacks and bolts as a clamp.
Last night I placed fillets around the core, in preparation for laying a glass inner skin.