I have successfully had a winter garden for salads the last 2 years. Next to one of my southwest patios is a concrete retaining wall. I put a raised bed next to the retaining wall.
I bought a little white wire fence on sale from Walmart for $2.00, put it around the other three sides. I anchored a double layer of floating row covers on top of the retaining wall, cover the bed, and anchor it next to the fence with logs from our firewood supply.
The concrete heats up in the daylight, and release heat at night. Sometimes I also line the sides with milk jugs full of water. I add straw and other cover as needed.
To access, I lift up the row cover on the retaining wall for access. Very cheap. Everything except the fence was on hand already, and everything was reusable. It’s sort of a cold frame type of arrangement using row cover.
Carrots, spinach, lettuce, green onions, and garlic have grown well. I also have perennials this year in another such bed Rosemary, Stevia, tarragon, and lilac so we’ll see how it goes.
Thank you. The reminder to consider thermal mass will help very much (water, concrete).