Come on, you do not need marine plywood...the glue in marine plywood and exterior plywood is the same...marine plywood is stronger because it has more plys and little or no voids..strength is not the issue for what you are doing...you just need something to put some screws into without the screw going thru the hull. Liquid nails would work great and a quick coat of paint so that it looks nice then attach the electrical components.