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Fiberglass or concret seams to be the only logical thing. Fiberglass would be by far my first choice. You could build everything up with ripped down PVC and a heat gun, secure it to ply, then mold the fiberglass or concrete on top of that. Its going to weigh a pretty good deal, but it will work and should take paint and the abuse very well. Any type of foam will not make it, i don't care how much foam coat you use.