Below you will see Anatomy of Cold Frame
Consider these factors when you building a box to extend the growing season.
Site Surface: Frames work best if the top is angled slightly toward the winter sun. You can either cut slanted sides or mound soil to make the back edge of the frame sit slightly higher than the front.
Frame: Scrap wood or untreated 2 by 4 or 2 by 6 pine boards are fine. You can upgrade to rot resistant cedar, redwood or locust, or composite plastic lumber. Other option include logs, baled hay or straw, bricks or concrete blocks.
Corners: If you only have a handsaw, a hammer and a screwdriver, you can build a sturdy box from 2 by 4s crews and four steel corner brackets. Brackets come in different forms some for inside the box and some for outside. The simplest ones screw into the top of a frame that’s already been banged together with 3 inch box nails.
Covers: The best materials for topping cold frames are tempered glass patio doors or shower doors, which are often discarded during remodeling projects. Heavy enough to resist strong winds, shatter resistant tempered glass doors are better than standard storm window or paned windowsm, which can be a safety hazard. Look for doors that still have hardware attached and leave it intact. Later after you have build a frame the existing hardware may prove handy as part of a hinge or a ready made handle. Tempered glass door come in all sorts of weird sizes so it’s best to source a top first and then tailor the frame to match it dimensions. Use thick blankets to bring winter sown frames through winter storms or to block sun if you can not be around to vent the frames.