As you may recall, I added a factory rib to the left rear and fabed up my own for the right rear. The back edge of both side walls are screwed into these vertical supports. A couple of months ago I cut two pieces of 1"x1" angle steel and ground the edges until they fit as flush as possible.
I'm gonna weld them into place from hinge pillar to the ribs and see if they will give the needed support.