The Man is talking "polyurethane" foam, NOT polystyrene. Polyurethane foam is excellent for making plugs for fiberglass parts, etc. It will not melt when glassed w/fiberglass resin. Check some of the home supply stores or where a/c-heating units are sold. Here, in east coast S.C. a 1"X4'X8' sheet is under $20.00. You could keep an eye out for local schools re-roofing old schools. I lucked up on 180 sheets, 3" thick just for hauling it off & the contractor didn't have to pay landfill fees. Foam on. Rick