I have not done it in a Mainship but I did it in our last boat. Pretty easy. Cut a piece of plywood to fit. Epoxy coat it. Mix up some thickened epoxy and trowel it in the locker where the plywood goes. Set the plywood in and put enough thickened epoxy on to make a nice filet on both sides. Then use something like strips of 1708 to glass the joint. If you let the epoxy go off during the process you will need to wash off the amine blush that forms on the epoxy. Scrub brush and water will do it.