I made a (much smaller) box like that out of door-skin on a 1 x 2 frame (cut down 2x4) and coated inside/outside in cheap polyester resin and glassed the edges.
Did what Rolf said and cut larger hole for screws, filled them in with epoxy/cabosil and re-drilled them. Learned that from mounting winches on my dad's boat.
Thing went 3 seasons (at least) without noticable rotting before I sold the thing. Stored outside and washed the boat on the trailer occasionally...
Probably would have lasted longer if I'd designed it better. Could stand on most parts of the box, too.
