How about an el-cheapo "lean-to greenhouse" on a south facing wall?
This one was made from scrap (free) PVC pipe & $15 worth of plastic sheeting & duct tape.
We did cheat a little with the addition of a box fan on a thermostat that draws "warm" air from under the house.
zone 9 & it never gets below 40°f (that's as low as the thermostat goes)