I don't have anything to sell you but wanted to share some inexpensive ideas with you regarding a cat box. Its kind of crude but cheap.

For storing extra hardware, sails, rope, bungees, etc. I used a plastic cab box for a pickup truck(the type that goes behind the rear window.) It has 2 lips on each sides that were meant for the pickup sides, but screwed down the bottom to the frame of the trailer. The one I bought was from a rummage sail and was in great condition except it just needed a power wash.

For storing my sails I used a 12" diameter x 10' long plastic corrogaged drain pipe and drilled 1/4" holes in the bottom of each "rib" to drain any moisture. The "ribs" on the inside also keep the bottom of the sail dry and has worked great. On the back end I'm going to add a large plastic flower pot to keep the sail from sliding or blowing out. The only thing I don't like about it is the pipe has to be supported every 2 feet from underneath or it will start to sag.