This isn't a massively big cold frame, but it's roomy enough to fit ten pots of peppers. My dad built it for me from a pattern I found in an ancient gardening magazine. You only need one sheet of plywood, some hinges, and some plastic (for the lid insets).
We've had a few cold mornings lately, with the temperatures dropping to 10ºC (and once or twice even lower than that), so I've been watching the plants in this cold frame closely to see how they are handling the cold. So far only the basil is having a tough time. The peppers, as long as I barely water them, are fine. I even have more peppers growing, but the newest hot peppers have no bite to speak of - they need the summer heat.
My eventual intention is to put the salad mint and some lettuces in here this winter. If it gets really cold I might put in a heat mat, but I want to see just what can be grown in this thing.