I made a half tonneau for my truck. The first photo shows it in the forward position where it wouldn't normally be of course. Maybe others can profit from my mistakes...
The Rambox has an aluminum channel at the top, on both sides. The channel has a groove on the top surface and another on the side surface; I used the latter for this project.
I found some old used cedar 2x6 decking in my garage and cut two pieces to length. These would be my support; you can see them laid flat at the bottom, in the photos. The length turned out to be a problem because the Rambox channels curve in together at the middle of the box, and spread out a bit at the ends. So, the length is determined at the middle of the box, and then a little bit shorter to clear the screws inside the channel. This will leave enough length so that when slid to the ends of the channel, there will still be some engagement.
Then I cut basically a dado (using my radial arm saw) to get the length at the top of the 2x6, which is less than the overall length determined above, and again is determined at the middle of the channel where it is shortest. Try to get this length to just barely fit between the channels at the narrowest point.
The channel end caps are removed, and the two boards are slid in. Figure out where you want the front one, and the rear is all the way back with the tailgate closed. This front position will depend on where the notches are for the tie-downs; you want it to come just behind a tie-down that is located in one of the notches. Unfortunately I did not figure this out until I was done, so I have a gap of about one inch (see middle photo) which allows the tonneau to slide forward when I hit the brakes. I will have to fill that gap with a shim of some kind to stop this forward movement.
Your guess is as good as mine how long the tonneau should be. I figured since a vortex is going to be circulating forward of it, the height of the vortex should be about the same as the fore-to-aft dimension, so the bed to rooftop height of the opening is about the same as the dimension between the front of the bed top and the tonneau, and the tonneau is rearward of that filling the rest of the bed. Actually I made it too long the first try, so I just cut some more off for the second try.
The width on the other hand is just about as wide as you can fit in between the Rambox covers. When you open one of these covers it does not protrude any further into this area. Unfortunately this is around 52 inches which means you have to consume a whole sheet of half-inch plywood to do it. I later thought that I could have made the tonneau with two pieces rather than one which would allow one to economize on plywood. Yeah I am cheap!
I decided to shim it up a bit to get the height to match the height of the tailgate; you can see the shims in the pictures.
I painted the top with deck paint. It's not very pretty so some might want to try another way to get a smoother surface, and colored black like the bed too.
The nice thing about this is that it is strong enough to hold my weight so I can scramble over the tonneau to the front of the bed. It can easily be slid to the front of the bed, or slid back off the channel entirely and thrown into the bed in case you are carrying something large. It's a good idea to be careful of the materials you use so moving it around like this won't give you a hernia.