I have discovered that an old chest freezer makes a good cold frame to start seeds.
I place 5-gallon buckets filled with water, which act as thermal mass, in the bottom of the freezer. On top of the buckets, I set the trays of soil and seeds on old refrigerator shelves. I tape plastic sheeting to the top back edge of the freezer, under the lid. For the front and sides, I fold over the sheeting to make sleeves to hold metal pipes that keep the plastic from blowing off.
When it's warm and sunny, I open the freezer lid and fold the plastic back. When it's near freezing, I reposition the plastic and close the freezer lid. Rain has filled about a third of the freezer, but I have left it for additional thermal mass. The extra humidity seems to help the sprouts, and I don't have to carry water from the faucet to water the flats.
St. Paul, Arkansas