I tried a galvanized steel box on the back yesterday, but it makes a loud booming noise when I go over any little bump, so I have to think about that as well. I'm open to ideas.
Stress the flat areas and that will stop the booming however it is still a sound box and will make noise unless loaded. I had a fiberglass sailboat that did that when new and empty but after I got it loaded it pretty much stopped. I finally stressed the hull on the inside with some flat stringers fiberglassed to the hull to stop the oil canning. I was told that if I just filled the hull with money and tamped it down that would stop the drumming and I believe that would have been cheaper in the long run.
You might try attaching rubber floor mat material with silicone to the inside to dampen the vibration of the flat side and bottom.
I admire the engineering and intend to copycat.
