I bolted a pallet to the bottom of the carryall "L". Then I bolted some 2by4s around the edges and affixed other 2by4s to them to make standards for side pieces to fit into. Only attached some 1bys to the vertical part of the "L".
Attached are a couple of pictures.
For the back side, I put a hinge in about the middle of it. You see it folded down in the one picture.
Ralph