I've built two different boxes out of wood. (Two different boats and trailers). The first was bascially 3/4 inch marine plywood. Treated 2x4's, SS screws, etc.

Trailer shapes are basically a rectangle with a triangle, or trapezoid on the front end of it.

I always built using all available space, as in the rectangle 4' x 8', by 12" high, with an additional 4' base, by 4' sides, by ~ 2ft front trapezoid. You can keep 2 set of sails, several poles, extra everything, etc. in a bos with this much Sq. Ft.

By second box was built much lighter, out of of 2x2's frame 12' long, x 4' wide (fender to fender), by 12" tall, A series of two doors, that always stayed relatively water tight. I used T=111 treated siding sheets, lasted for years, never rotted. It had a spine in the middle, so you could easily stand on it, dance on it, etc. The guy I sold it to took it off, not really sure why, it was very functional.
It was the the 'table' at many a regatta on the Tx Gulf coast for skippers meetings, award ceremonies, and many an after regatta dinner table, etc.

I always strapped the cat wheels to the bows, underneath in front of the box, lock to the trailer, an never had a problem with it.

You have had some great suggestions, go for it!