Years ago I carried test equipment and assorted tools and a ladder in a topper that has hinged side doors instead of side windows. I split a piece of plywood and stood the two 2'x8' pieces on end running cab to tailgate. 2x4s tied them together. The bottom boards held the test equipment cases (about 3' square) from sliding front to back and the top boards let the ladder ride up and out of the way. Openening the side doors revealed flat 'walls' with wrenches, ext cords, etc were hung on hooks and room front and back of the wheel sells for tool bags etc.

Maybe this will give you an idea.