a. There is not enough space for thick rigid panels
b. You can't lay semi-flexible panels
over a surface with ridges without shortening their life (fatigue from repeated flexing).
Has anyone bonded the panel to a backing sheet of something like 1/8" FRP, pre-bent to the correct curve? It would add negligible weight and thickness, would stiffen the panel by at least 4x, would still allow a removable mounting, and should lengthen panel life. Other backing materials are possible.
Seems simple. Other than the time, there seems little down side.