A solar greenhouse can be made with almost any kind of closure that lets sunlight in at least the south side of the container. Suggestions on containers especially a insulated one.. Even a raised bed that has a extended top for winter and a double clear cover like a glass with a second or even a third cover will make a nice solar greenhouse with water containers inside. a dozen gallon milk containers inside the cold frame will work.
A even better solar type greenhouse could contain a light bulb that would be connected to a thermostat that kicks on only when temp approaches 32 degrees. Lettuce would grow all winter even in a 16 square foot solar greenhouse.! Put those saved flowers in a cold frame to keep all winter! Tripple covers really work great! Lets hear some suggestions on enclosures like evena 50 gallon drum set on its side?