Chaps. Not sure if this is the best area for this, feel free to move if somewhere better. I need to fit some kind of lockable storage tube behind the cab of my wagon, something along the lines of a Rhino tube that plumbers have on their van roofs. It will be used solely for storing my extending height measuring pole and a 4’ spirit level, and possibly a truck sized torque wrench, if ever I can acquire one , and needs to be just over 4’ long and 4.5”-ish square to fit perfectly in a gap just forward of the fifth wheel subframe. Have looked at various tubes online, but the prices frightened the bejesus out of me, so a homemade affair out of aluminium or m/s steel box will more than suffice, as long as it can be clamped securely in place and fitted with a locking “flap” at one end. Any suggestions or thoughts on design/availability of stock would be muchly appreciated.