We have been using stryrofoam, or expanded polystyrene, for about
8 years. We use 3/4 in., high density because it is durable, tried 1/2 in.
low density but the bees chewed it up quickly. A 16X20 inch piece is
placed over an inverted inner cover. The inner cover has a 2 in.piece of
the rim removed for ventilation.