Coldframes are great garden tools. You can use a coldframe to extend the growing season, because they act like mini greenhouses. So you can start plants early in the spring and you can keep salad greens growing all winter long.
I have a coldframe that is made from a recycled door. The door has safety glass, and the handle makes a convenient way to open and close the top. Other options include using a discarded window frame, or simply stretching some plastic over bales of straw.
The base of my coldframe is made with three treated 2x12's that I bought in 10-foot lengths. One length of wood goes in the front, and two are stacked and nailed together for the back so that the coldframe is angled. Raising the back of the frame helps water run off and it captures the most sunlight. It's also a good idea to orient the front facing south to catch more light.
The 10-foot lengths of wood works out just perfectly for the door, it's about seven feet long, and that leaves three-foot pieces for the sides. One of the three-foot pieces has to be cut at a diagonal to get that 90-degree angle. Of course you have to measure accurately to get it even, so plan on that. And you'll need some extra scrap pieces of wood to add support at the corners.
On especially sunny days, you have to open the top of the frame to allow air circulation so that your plants don't get too hot. I keep an old piece of post handy to prop open the door.