Probably NOT 56k friendly though...
notice my highly elaborate height adjustment device (aka scrap of treated 1x1)
up inside view
The tonneau is recycled from my original (don't remember the brand name) that came with aluminum rails & plastic snaps.
The seams started ripping off the snaps & it was a pain to unsnap & roll it up so I made a treated frame that sits just inside the bed rail protectors.
The cover sets about 1/4" above the bed sides & when closed, it fits snug up against the bottom lip of the toolbox.
Construction is pretty simple, 1"x1" treated decking for the frame (rounded edges over to eliminate snags), the corner braces are more treated decking scraps. Glued & screwed together with exterior glue & screws.
I even re-used the old aluminum bars to add a little curve to the top to keep snow & ice from building up on the top.
A couple of carriage bolts up front for the blocks to pivot on.
The blocks near the tailgate have a bolt that slides through the bedside lip & hold it in place. The bungee cords hold the back side of the cover down when I have wood or something else that won't fit under the cover.
It takes less than a minute to pull out the bolts on the back side (tailgate) and slide the cover back off the front bolts.
I use the 2x4's to support 4x8 plywood until I raise the tailgate.
The back one keeps groceries from sliding all the way up to the toolbox.
It ain't pretty underneath but it works perfectly for what I need.