I have decided to try extending the growing season this year. It is quite easy to do so in SFG. Materials you will need:
1. 6 mil poly, or vapour barrier. Usually comes in a row.
2. 2x 4′ long 1/2″ PVC pipe flex. I have used plastic electrical conduit as well but pipe flex is a little bit cheaper and easier to bend.
3. some bricks or stones.

You bend the pipe flex and stick the end into each corner of your SFG bed. Then attach the 6 mil poly on top. You can use clothespin to clamp the poly with the pipe. Then use the bricks to hold down all the sides. In winter, the wind is harsh so make sure you hold down the poly securely. During a warm day, you should open a corner and let some hot air escapes.
What you plant in this small greenhouse also plays a role. Rememebr to choose cool veggies meaning lettuce, spinach, swiss chard, green onion, carrot, radish, etc.

Each SFG bed can be setup as an indiviual greenhouse.
Holding down with brick and stone.
Use a cable tie at the center to attach the pipes.