The interior fabric-covered wall goes up under the window's clamp ring, making it look nice.
It's difficult for me to get to an edge of that wall right now (everything in the way), but I think it might be 3/16" plywood plus the fabric. Unfortunately, I can't get under it, so I can't see how thick the wood support around the window is. I'd say you're thinking correctly, with the caveat that I haven't done anything around the window, myself, yet, so I have no actual experience here
After replacing the damaged pieces with new 1/2" pieces, I placed a 19/32" sheet of 4'x8' sheathing and secured it to the wall using 1 1/8" screws. I then used a jig saw to cut the hole out from the outside. I then placed a 5mm thick sheet of wood over the sheathing with wood glue, finish nails, and a few wood screws near the top where the fiberglass curves slightly. Stacking these pieces together gave my wall a total of 1.5" thickness.
Lastly, I placed some butyl tape on the flange of the window and secured it into place with the ring clamp and new metal screws.
