If you don't want to drill holes in the sides of the box for some kind of perimeter frame for it to set on, your only other option is to build up something from the bed of the box, like for instance a 3/4" plywood rectangle that just sets inside the existing box and extends upward untill it reaches the proper height to support the cover. I f I was doing that I would hinge the cover along the front of the box so that you could lift it up at the tailgate end to carry stuff in the back of the truck under the tonneau cover.