A simple wooden-framed box, insulated on five sides, with a sheet of readily available safety glass (like for sliding glass doors) on one large face. Mount the box to a frame, facing south. The frame would be adjustable so the angle of the box could be adjusted to accommodate changing winter sun angles, similar to PV mounts. Two insulated ducts would be connected to the box, one at the top and one at the bottom. These ducts would carry air from the house to and from the box. The air would get heated in the box, thus providing warm air for the house.