I am in the process of replacing the transom on my 14' aluminum boat. I removed the old and rotten one. There was only a board on the inside. I am thinking of putting one on the outside and inside. Will only be using a trolling motor, but maybe an outboard someday. I have scrap composite decking lying around. I am going to try and use that on the inside. If I can't get it to work, ill seal wood. I am definitely going to use composite decking on the outside, unless it will cause corrosion. How does this sound?

Do yourself a favor. As you are working on this project send a pm to Kris2016

He is my buddy and he did a restore that he would be happy to share with you.  I know he has it listed on several boards but would share the links and discuss what he did. He resurrected a great boat after a bunch of work.

You can use se 3/4" marine plywood, stain it whatever color you like and seal it Varathene/polyurethane.

Tom.