You may want to consider building a nice cold frame instead. You can make a great one by re-purposing an old storm door, patio door, or window. The volume in a cold frame is small enough to be heated by one or two light bulbs, at least down to the mid 20's. Face the cold frame to the south, and place it beside a heated building if possible. If only the top is made of glass, then angle the top so that the sunlight hits it more directly.