I used 2 pieces of 3/4 plywood glued together with Gorilla glue, it is waterproof.
I started with marine grade plywood but you aren't likely going to find that so go with exterior grade, once your peices are cut out and the glue is dry I would paint them with a basement waterproofer type product,Dri-lock is one kind I have seen do the front back and sides then bolt it in.
Thompsons is good for a deck on your house but I wouldn't use it on a transom board.
Use stainless hardware if at all possible and when you cut the boards start big and trim to fit.
Good luck
When I replaced my transom I used 5150's method, with the exception I used non-corrosive screws to hold the 2 pieces together along with glue.
Also I took the extra time to waterproof the plywood with 2 coats of resin. I drilled all my holes before I painted on the resin.
Use your old transom board as a pattern, cut the new board a little larger and trim to fit.
Thanks for the info lots of help. If I go with the Seacast will it last longer an be stronger than just wood? Can you drill through the Seacast? What do you all think about using Gluvit for the inside the hull before redecking? Thanks again.
Yes it will be way more gooder than wood
Be sure to follow the instructions letter by letter there is a manual on how to on the site also look at the FAQS listings.
Gluvit works. Good stay away from pressure treated woods around aluminum