Made this box in 2004 in northern Minnesota Drove it west to Seattle with a side trip south to jtree in January/February as a moving road trip. It was packed to the max although I do not know the exact weight. I still use it as a storage box.
I just went to the hardware store bought some 2x2's for the frame, a sheet of marine grade plywood, a can of primer, and some caulk to fill gaps. Just went to work after some simple drawings and a little thought.
You should give it a go and see what you get.