I have used an engine mount which articulates to bring the motor up and out of the water. But on other transoms where I wanted to protect the fiberglass, I have either used thin plywood, which I coated with epoxy and then a couple of layers of polyurethane varnish to give UV protection, or one of the rubber transom protectors.
Something like an old car rubber floormat or similar can be cut to fit and work pretty well also. I used a clear piece of rubber carpet runner material on our Lund and it worked great.