A cold frame is an enclosure used in Northern climates to "Harden off" tender plants on their way from the protection of the greenhouse environment to the outdoor planting bed. An aluminum Storm Door makes a good top. It already has hinges to allow easy opening for ventilation, and it may also have a sliding glass which will help control its environment. A Thermometer is a helpful addition.
It generally faces South angled toward the Sun, and has Northern protection in the form of a wall or shrubbery to help protect it from cold winds. It may need a light bulb or other heat source to keep it above freezing on cold nights. A tarp or other cover is also helpful. If you are able to place it against a heated building, an air vent between the two provides an easy method of controlling the temperature.
The sides can be made of wooden planks, landscaping timber, railroad ties, poured concrete, concrete blocks, bricks or stones, aluminum roofing, or many other materials. Materials that are painted black will absorb additional heat. Masonry materials will retain the heat longer than wood or metal. The front should be as high as the tallest plant you expect to keep there. The back is higher to allow maximum sunlight.
Put tender plants in it about 2 weeks before planting time. Water them regularly, especially if the plants are in pots as opposed to ground plantings. Gradually open the cover on warm days to get the plants accustomed to the outdoors. Automatic Openers/ Closers can be used to ventilate Cold Frames on Sunny days.
A portable Cloche can serve the same general purpose, but it is usually used over plants in the garden while there is still danger of frost. Care must be taken so it has ventilation on sunny days to avoid Sun-burned plants.
A similar Cold Frame is made by replacing the top with strong plastic strips. Or the strips can be fastened to a removable frame. The strips are about 6" wide and go from end to end. They can be stapled to wooden strips which are then fastened to the ends. It may be necessary to add a center support as show above to keep the plastic taut.
They have 1" spaces between them to allow ventilation and rain, but still allow enough protection to raise the inside temperature and grow cold-weather plants or harden seedlings. The spaces allow a hand to tend or harvest plants. A tarp or insulating cover may be necessary with this design.