Banana wrap? The curved part that wraps from the lower body trim around underneath? If so the ones for the trailers are available from Airstream, I would try there or a salvage yard.
I've done it. Get a cheap plastic welder from harbor freight. Melt fuse the breaks together. Use plastic caps from spray paint cans and similar to reinforce, fill in, level. Melt in or onto where needed. Use hand grinder to smooth. Then switch to standard body fillers. Follow standard body panel finishing from there. For classic paint match, tool box grey from sears.
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