That's something I have thought of but have not had the nerve to try. Plexiglass is going to quickly scratch up. Maybe you can get some lexan. Bake it in the oven slowly to make it pliable, then form it to the curvature of the hull. I would put a couple of wood strips running along the keel line across the window as a sacrificial rub strip. I also would gasket it and screw it in so it's replaceable yet waterproof. The only sucky part is if all this fails you have a boat with a hole in the wrong end. It would make rolling easier- just roll over, pop your head through the bottom hole, and keep paddling! :-)