I built this cold frame out of scrap lumber, plexiglass, and foam insulation. It’s worked surprisingly well — I’ve been able to grow winter kale and winter spinach in it, even in January, and it’s warm enough in the late winter to start onions from seed.
I was curious about the temperatures in the frame, so I bought a cheap wireless thermometer and monitored the temperatures over a few weeks in late winter. The temperature swings were wild, and it was losing almost all of its heat overnight, so I took a bunch of large rocks and put them inside the cold frame (not pictured). This mitigated the worst of the swings, as you can see from the graph below.
I’ve used this for several seasons now to jumpstart a few different crops, and during the main growing season I plant a few vegetables in it to make the most of the space.