Here is an experiment that I consider suggestive, if not successful:
In each case, two (edge-rolled) pieces joined. One rolled across and one with the seam. The idea is to not have flat pieces joined with a framing member to prevent water penetration and improve the aerodynamics.
I used scraps of Polymetal and I just braked the material without scoring the line which broke the outer aluminum skin, resulting in a seriously weaken result. Sheet metal or scored Polymetal would have been more successful. A strap or T-section could be inserted between the pieces for strength or interior attachments.
The piece with the seam running across the roll uses a plastic H-channel and superglue for proof of concept. A final version would need a deeper H-channel and rivets or something. I just worked with what I had on hand. Larger pieces would need an edge roller rather than the roller/brake/shear I had available.
HTH