First a disclaimer...I have not "done" much to my FP, but I have spent hours measuring, cogitating, and doodling alternative solutions to what FRP builds and even what AT specs. I did design and help to manufacture an "aftermarket" fly with windows, drawstring, attachment loops...an overall better solution to the FRP fly. So, these are nothing more than the ramblings of a mad camper.
The bottom line for me is that I am not overly impressed with the durability of the FRP FP. (Never even seen an AT one.) So, I started thinking about reinforcing mine with fiberglass and quickly moved toward reinforcing it with a tube frame skeleton, internal or external.
Quick digression: I bought two trucks to fit my FP. Imagine that. Rather than get the truck I want and finding/buying a FP to fit it, I bought a used FP and then shopped for a truck to fit the FP. In many respects, I feel stuck in a full-size short-bed truck; I think I would prefer a Ford Ranger... But that speaks to the rarity and uniqueness of the fundamental concept and several thoughtfully designed components.
So, I spend lots of time thinking about this: what vehicle would be my first choice and how would I modify my FP to fit it. Here's where I net out (at least today): what makes the FP unique is the hinge, torsion bar, lid, and tent design. Like I said above, I am not overly impressed with the construction of the shell itself. And the tent/pole set-up would work far better if there were a way to create space between the tent and fly (or design a waterproof tent).
I think the hinge design does not really lend itself to installation in a fiberglass shell, either. So, why not build the lower section of the shell out of mild steel tubing, skin it with aluminum, wire it, insulate it, finish the interior and attach the hinges and lid to it. I could make it to fit any truck bed I wanted. I could even start with a lumber or ladder rack as a foundation and build barn doors, rather than clamshell ones, into the tailgate area.
This sounds like a huge project, but the lid, tent, and fly of every FP is the same size. So the only lower shell section needs to be fabricated. And, like I said, a lumber rack or even a steel work shell could make a good starting point.