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I'm going to 'find' a political sign to make a sample fender skirt with; but no reason not to share here. Sorry there's no picture, but imagine a skirt cut out of coroplast with the flutes running horizontally. The internal ribs are notched a fraction of an inch at each end. Then a piece of welding rod, butt-joined if necessary for length, is bent to fit and another piece is slid through the bottom flute. They are joined (I'm a little weak on detail here). The open edge of the cut coroplast is pinched together and wrapped with metal tape. Then the bottom piece of welding rod (inside the coroplast) is bent to the necessary contour.
Fastening is left as an exercise, but a washer (ground flat on the edge) could be brazed to the rod and fit into a hollow in the coroplast to make a hard attachment point.