Sorry about the late reply. My excuse is I just re-visited this thread after backpacking Mt. Moriah in eastern Nevada, Great hike.

Anyway, I like several of the ideas:
1. using "caps" of light nylon instead of a full cover on the back side OR the "caps" that are connected to the bottom (seat) piece by a slim, curved piece down the middle & widens out to the bottom piece. (Can ya picture that?)
2. using carbon fiber rods from a kite store
3. maybe an alternative "pack-as-a-backrest" would be the UL path, W/ adjustable nylon straps from the pack to the pad. (sound of gears turning in head)

In any case I insist on a full under-seat cover to protect the pad from punctures. (Yeah, I know, "Good luck on the puncture thing.")

It ain't that I haven't made my own Thermarest chair cover years ago but I made a FULLY covered 3/4 length mattress cover from a too-heavy polyester fabric.

Thanks very much guys. If I get this underway successfully I'll attatch pics of it and measurements.

Eric