In the basic model commonly available transparent sheet polyethylene was stapled to a 500 mm by 500 mm frame made from 40 mm by 20 mm battens (like a double-glazed window with an effective aperture of about 0.25 m2), which was placed over the cavity. The cooking vessel was a 200 mm square pressed steel pan 50 mm deep. A small sheet of the same plastic formed a lid to the pan to maximize direct solar gain to the pan and to minimize heat loss by evaporation/convection.