[Wavery]

No matter what, all of the rotted wood will have to be cut out. There is no "easy fix" to wood-rot.

Wood-rot is a living organism that is feeding off the the good wood. As it destroys the good wood (thus rotten wood) it keeps moving on to find nourishment.

It really isn't hard to take apart the lower edge of the box by pulling out the staples etc to expose the wood. Just keep cutting out rotted wood until you reach good wood. It can be replaced with 2x4" aluminum tubing. I think that I would pop rivet the skin to the tubing about every 3-4". I would find where there is vertical frame members that I could tie into. Maybe get some 1" x 3/16" flat bar aluminum to screw in all along the lower edge to give added strength.

Just my thoughts.........I haven't done it.