First step is to attach 1/4" x 4' x 8' Birch plywood panel (trimmed down a bit, so actually about 7.5') to the lower 4' of the wall.
Next we measured the distance from the top of the 4' x 8' to the roof -- repeating every 2". We then transferred these measurements to a large piece of cardboard and connected the resulting dots with a best-fit curve. The results were 95% right from the start -- only some minor trimming with sandpaper was required.
To add support to the upper edge of the 1/8" plywood, we attached scraps of 1" x 3" to the wall. These provided an extra nailing surface for panel attachment.
We traced the contour from the cardboard template; I got jiggy with the jigsaw and the results were pretty good!
The wiring harness poking out of the upper right corner of the wall are the wires for the brake lights, rear turn signals, reverse lights and license plate light. We'll chuck these into some conduit at some point, probably run along the wall on the 1" x 6" pine board.