In answer; yes, and it's worth a picture—let me get back to you.
I guess it is coroplast. I thought it looked like corrugated cardboard. Was it spray painted? I guesss the next step up would be
Alumacorr.
EDIT: Will this do for now?
From the upper left, raw coroplast, V-notched so the wire or rod will stay centered (a flat-bottomed notch won't do that), and the V-notch recessed so the material can be rolled over the edge. The middle one could be used with a taped edge (packing tape or metal tape), But the recessed version could be rolled outside to insde over inside to outside and you'd have double thickness of the material on the edge and lots of area for gluing.
As for the washer, you could roll the wire around the edge of the washer and solder or epoxy it. then gouge out a cavity in the flutes and when the wire is threaded through it will pull the washer in with it. If nothing else, it's less parts to lose alongside the road on a dark and rainy night.