Foam selection is pretty critical. Some materials are difficult to carve and shape, or might fragment and flake. Many types of foam will boil or melt in reaction to fiberglass or epoxy resins. We tested various foams with both resins to determine reactive characteristics and yellowing tendencies under UV sunlight.
This material is craft foam found in the school supply section of WalMart or Hobby Lobby. Of course it was the most expensive option.