I don't have my Peak in front of me right now, does it have the velcro to seal in or allow the foam "frame" to be removed like the Jam/pinnacle? if so, I would think that would be a great place to slide any sort of frame into. I would look at delrin rod (available through McMaster-Carr) or potentially a thin Kydex, which could be cut in an X shape (or any shape you want) and has the advantage of being easily thermoformable to your back or specialized shapes. If you are interested in Kydex, I have a size large Peak and some leftover sheets of .08 (on the thicker side) and would be happy to build you something. Maybe I'll whip something up tonight and post it to see what folks think!