Here is a picture looking toward the front driver's side from the back.
You can see the strips of 1/4" plywood used to provide support for the curve, and the back of the aluminum tape that spans the gaps providing support for the plastic on the outside. The top and sides are solid pieces of 1/4" ply, the corners are strips.
The plastic is wrapped around the outside, then stapled to the 2x4 frame and you can see some of the excess hanging and tacked up out of the way in the image. It could be trimmed down to the 2x4 - but I was in a hurry. The osb board along the side was used to provide support and structure to the osb ribs before the plywood was attached.
The 2x4 in the middle of the image just ties the two sides together at the midpoint and provides strength when taking it off and on.
PS - it is pretty stout - during the ABA testing, a ramp I was trying to use to climb up to the bed came loose and I had to stop its fall with my head. It ended up bouncing off the ground (and leaving its marks on my head
). The structure didn't suffer, but a bit of the plastic was worn off the bottom.