Many years ago when I had a cap on my truck I built a frame out of PT 2x4s wide enough so I could back the bed of the truck into it. It had shelf brackets mounted on the supports, mounted so they were parallel to the truck bed. I'd back the truck into the frame, unfasten the cap, then lift up on one end slide a 10' 2x4 across from one side to another...one at the front, the other at the back then lift all 4 sides onto the shelf brackets (which were set up to give ~6" of clearance to the bed sides). Worked like a champ. Sorry, no pictures.