Thermal mass is your friend. Milk jugs filled with water, for a start. Maybe painted black and put in the sunshine... https://en.wikipedia.org/wiki/Trombe_wa

Thermal mass is your friend. Milk jugs filled with water, for a start. Maybe painted black and put in the sunshine... https://en.wikipedia.org/wiki/Trombe_wall https://en.wikipedia.org/wiki/Trombe_wall