Background: worked in engineering department for a leading composites manufacture of parts for medical, wind energy, mass transit and ag. equipment

i think fiberglass frame sheets would work great. to start you will need to find a resin that is flexible. use sheets of fiberglass mat for your structure then saturate the mat with resin. you can get mat that is woven like fabric or felted ( lots of random strands in different directions) the different woven mats provide for strength or flexibility in different directions and by layering them you can create a sheet with any characteristics you desire.

i also like the granite gear packs and own a latitude vapor, but their frame sheet is not really a composite or a fiber reinforced plastic, it is a solid thermoplastic sheet with different holes punched in it to create flexible and stiff areas, possibly with some solid filler material in the matrix, still a great design but not quite the same.

any questions on how to work with fiberglass let me know.