See great tips for building a box to extend the growing season in winter
Site Surface: Frames work best if the top is angled slightly toward the winter sun. You can either cut slanted sides or mound soild to make the back edge of the frame sit slightly higher than the front.
Frme: 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 options include logs, baled hay or straw, bricks or concrete blocks.
Corners: If you only have a handsaw, happmer and screwdriver you can build a sturdy box from 2 by 4, screws and four steel corner brackets. Brackets come in different forms. Some for inside the box and some for outside. The simplest and cheapest one screw into the top of a frame that already beed banged together with 3 inch box nails.
Cover: 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 windows or paned windows which can be a safety hazards. Check with thrift stores. Look for the 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 ta top first and then tailor the frame to match its dimensions. Use thick blankets to bring winter sown frames though winter storms or to block sun if you can’t be around to bent the frames Snow make a good insulating cover.