Every gardener dreams of extending the growing season. You can do that by constructing a cold frame. One caveat about cold frames---you need to monitor the air temperature as well as the temperature inside the cold frame. On sunny days, it can get hot enough inside a cold frame to cook your plants, even with outside air temperatures in the 40s. If you can't be home to monitor your cold frame, install self-opening hinges. Most models of these hinges contain a cylinder filled with a liquid that expands and contracts with the surrounding temperature. When temperatures rise into the 40s, the liquid expands and the top opens. When the temperature drops, the liquid contracts and the top closes. These hinges are almost as easy to install as regular hinges.
Find an old window or patio door to serve as the top of your cold frame. Patio doors are best because they contain tempered safety glass that won't break into deadly shards like regular window glass does. You can also use a piece of window-grade plastic attached to a wood frame.
Build a box the size of your scavenged window or patio door. Use plywood or particle board and cut to size with a circular saw. The cold frame will be oriented so the long sides are on the north and south when you put the cold frame in the garden. Make the wall on the south side of the box six to 12 inches shorter than the back side to catch the rays of the low-in-the-sky fall and winter sun. Cut each short end diagonally so the front matches up with the height of the front wall and the back matches up with the height of the back wall.
Secure the corners of the plywood box with corner brackets. Attach the corner brackets to each board at each corner with wood screws.
Attach the self-opening hinges to the back wall of the cold frame box and the back edge of the window or patio door, following the instructions of the manufacturer of the hinges. Each self-opening hinge will lift approximately 15 lbs. If your scavenged window or patio door weighs 30 lbs., you will need two self-opening hinges, evenly spaced along the back wall of the cold frame. If your top is light enough to only require one self-opening hinge, place it in the middle of the back wall of the cold frame and install a regular door hinge at either end of the back wall.
Attach an "eye" from a hook and eye type door lock to the side of the top of the cold frame window or patio door about eight to 10 inches from the front of the cold frame. Pound a sturdy stake into the ground nearby and attach the "hook" part of the hook and eye door lock near the top of the stake. When high winds are present and it is necessary to leave the top of the cold frame open, place the hook that is on the stake into the eye that is on the cold frame's top. This will help keep the cover from slamming shut or blowing open in high winds. If you used an old patio door for the top of your cold frame, put another stake and hook and eye lock at the other side of the cold frame. While this step is optional, it is a recommended safety precaution.