Yes, you have a dilemma. The simplest solution pending the thickness of your wall is to get a aluminum wall mounting trim such as one used in Argosy. That is a formed double TT trim with approx. 1/4" capacity. Call Ryan at 319-235-1115 at RV Reviwe.
I know that he has Argosy for scrap.
The problem you are having is with the depth of the wall pocket. It does not follow the curvature of the wall because it is made for straight line. Even if you could form it for now somehow, come summer heat, it will buckle in all directions. If you could find one of heavier cross section and a shallow wall pocket, you could make it work. Even if you had aluminum TT trim with 1/4" deep pocket, the sides will buckle as you attempt to curve it. I am not aware of existence of a suitable trim for that application. If you order Argosy trim the shipping in that length will be an issue, but you can ask Ryan to cut it in half at the beginning of the curve for shipping. Since the trim is formed and it cannot be straightened. Sorry, that I could not be of more help to you. Thanks, "Boatdoc"