Great work, Dave. To your question about a frame:
I recently made a ~5800 cu inch basecamp load monster that included a frame made out of a sheet of HDPE and two aluminum tubes, bent into a slight S shape and zip-tied with tiny zip ties through slots in the plastic sheet. This allowed me to seat the aluminum tubes right under the load lifters (so the load lifters pulled down right onto the tubes and the transferred to load directly to the hip area, where they slotted into a little webbing tube). I've heard you can fill tubes with sand and then bend them (to prevent kinking), but I just carefully supported mine on the exterior and bent them slightly. No problems. And the things carries beautifully (please don't ban me from BPL based on these weights - they included lots of shared gear with non-ULers and mountaineering gear!): 40-50 lbs quite comfortably, and up around 60-65 the frame does well, but my shoulder straps aren't thick enough. You can get aluminum tubes and plastic sheets from McMaster Carr.
And again...nice work on the pack!