I built these cold frames this spring a la Four Season Harvest:
I'm really loving them. Just a 4 x 8 box with a hinged lid of glazing. I put them right onto grass and left them closed a few hot days and killed all the grass, then no-tilled my seed right into the stubble. They've survived many cold nights now. I had my freezer full of turnip greens by early June. I can regularly go out there and fill a grocery bag full of greens. There is a ton of chard, bok choi, mizuni, spinach, leeks, and vit in there right now. As silly as this sounds, the thing I used to go to the grocery store for the most was salad greens. I could grow some in warmer months, but colder months I was still dependent. I haven't been to the store now in ages. Greenhouse is the next project come spring.
They are the same size as my rabbit tractors which is nice because in the hottest weather I can pull the wired lids off the tractors (seen to the right) and just open the cold frames all the way and put the tractor lids on top to keep the chickens out. I only really worry about predators getting into the tractors in the colder months.