Small update on my too slow work on these mods:
I adapted my DIY copper clips from the last version of my wheel skirts. Here they are with new screws in them after taping the copper. The idea is that the hoot part will be pulled into the frame by the screw, and the skirt itself will be flush against the outside of the body, making a kind of clamp for the skirts.
Here is the temporary skirt, cardboard, for testing the shape and fit final will be in plastic sheet probably). I changed the leading edge from the first cardboard version for simplicity's sake. The trailing edge was changed too, size reduced. But the trailing end still needs further modification, I think, because it is hard to make it flush with the rear bumper cover, and I am trying to avoid drilling holes I might regret.
The black tape was just to mark where the clips would go on the wheel well.