To save weight, I just built my box out of "door skin". It's thin plywood used for those cheapie doors that are hollow. Then, just glassed over them. Designed doors on the ends that flap down so I could slide sails in from either end. Put some reinorcement (1" x 1" wood) on inside edges (except for bottom so I could slide stuff in/out) and glassed them.

I could stand on the top with that system, and it was lighter than the standard 1/4" plywood method. Also, it was cheap enough to replace every other year if need be (some of the edges start dry-rotting if you're not careful with the glass). Painted it white (flat exterior paint) to look nice.