Re: Treated wood ok?
It is very important to use marine grade plywood.This plywood is much denser than any other.
A good transom is one and a half inches thick,so a good way to make one is by gluing two pieces together with waterproof glue (gorilla glue-water proof).After having glued it,drive a good number of wood screws through to keep it together while the glue sets(and expands).
After it is al hard and dry,cut the shape of the transom out of your double thick marine ply.Now use resin epoxy on all the cut edges to makes them water proof.The rest you can just paint with a good quality wood paint,but paint it twice.
When you drill through your new transom,always daub the inside of the drill hole with epoxy to make sure water will not penetrate into the wood.
Good Luck