Using Seacast or similar with 5in" Tarnsom Increase

[DonV]

Seems to me you are not very experienced on this type of work. This is not something you want to do for your first try and have it not work out. I can see re-coring a hatch for your first try working with fiberglass, however a transom is a different and much more important undertaking. Have you checked around for pricing from a professional?? You are going in the right direction with the 25" transom for sure.....