When I wanted a trailer box, I talked to my boat's manufacturer. It turns out he already had a mold for one. He had given it to one of the sailors to make his own box. I talked over some modifications with the guy who had it and agreed to pay for materials in exchange for the prototype. The changes included a split top and a drain plug - and I'm happy with both. I moved the box to a double-stack trailer when I bought a second boat and it has room enough for boards, booms, tackle, and sails for both boats.

Regards,
Eric