Perhaps this will help:
How do you get a 15" tall, 1.5" thick plywood transom up into a space that is also 15" tall+/- and 1.5" thick+/- when there's a splashwell lip that sticks down 4"+/- making a slot you have to sneak the plywood up into.
Add to that the fact that you need adhesive PB/PL coating 100% of the contact area between fiberglass transom skin, fiberglass splashwell skin and the new plywood core. All while not scraping any of it off while sliding the 1.5" thick board into a slot that's not much wider then the plywood is thick.
Over your head, tucked up under the splashwell, how do you plan to grind, scrape and pry all the existing fiberglass and old wood out and ensure you've got it all sufficiently prepped for the new? It's a slot that's more then 4" tall by a little over 1.5" wide.....