I'm still designing a cap for Pablo's bed. It's a 7' x 5 1/2' flatbed with stake pockets. I'm also going to need full side skirts. It's just a little cold out there to get it done tonight.
I drew the horizontal line 4' straight level back from the front of the cap, then angled it down to end 8" above bed level 12" back from the back end of the bed. Then I did a spline curve from the front to the midpoint of each of the lines, and then to the back end of the sloped line. I think this will give me room for a decent side door. The sides will slope so that it's 45" wide (cab top width) in front and 48" wide in back. That will involve torturing the ply on the sides slightly, but it's within doable limits.
My plan is to cut two templates out of scrap and bend a properly trimmed 4x8 sheet over them, set the side pieces in place and mark them to that curve on their angles, and then put it together with some interior framing. I'm imagining a large hatch in the roof piece to make loading easy, but I haven't worked out those details. As I said, there'd also be a side door.