How to Build a Heat Box for $6, Heat your Home Free Doesn't look nice, but will heat a 500-1000 square foot home from the sun's energy. Amazing. Building Heat, House, Commercial Building, Building Direction
Also on these boards
The Zen of Passive Solar Heating Panel Design - Front and back of solar heating panel with double trap to allow for totally passive heating without accidental loss of heat. This could 2-layer design could be combined with the soda can heater idea as a simple way to improve its function.