My 2 cents worth:

It would probably be less work and less weight if you took a piece of 1/8" or 3/16" thick steel plate and mounted it on the transom using the mounting bolts to secure the plate to the transom which would also add more strength than the glassed plywood.

The plate would need to cover as much of the transom as possible to provide the most strength to the entire transom. If you really went all out you could flange the plate to match the sides of the boat and secure to the sides which would provide as much strength as possible, but that might be going a little overboard and I don't know how much interference you have mounted on the inside of the transom.