My project involves making a 5 ft*3 ft*4 ft (l*b*h) aluminium/stainless steel sheet metal box at the back of a slow moving pedal vehicle. There is no load on the sheet metal, except from small vibrations from road travel. My question is: 1. What gauge of sheet metal thickness would be enough to prevent buckling from its own weight? How do I calculate this without experience? 2. Would you recommend using a stiffener just for a higher factor of safety? If so, which type? As an aside: Can one join the aluminium box without welds/braze i.e just using hems, lock seams etc.? Thanks in advance!