Looking at the outer radius of U-shaped rear shroud, I found a piece of 6" sewer pipe (un-used) with roughly the same radius. Surely it was a simple matter of bending the panel around the pipe?
Having done this and presented the rear panel up to the chassis, I found that it wasn't such a simple process after all. The radius of the bend at the top of the panel was simply not tight enough and if I tried to tighten it the panel didn't fit around the rear uprights. I tried a number of bends but the panels needs to fit close to rear chassis member which limits movement.