Cut the Lumber for the Lid
Measure and cut two 2x2 pieces of lumber, each to a length of 39", and cut two 2x4 pieces of lumber, each to a length of 69-3/4". These pieces will make the casing for the cold frame lid. Measure and cut two 2x2s 32" long; they will serve as a support brace for the center of the lid. Form the pieces into the frame of the lid. Make sure that the outer dimensions of the lid are the same size as the outer dimensions of the base of the cold frame. Otherwise the lid and box won't line up.
Note: You can recycle an old window or glass-paned door to use as the lid for your cold frame. It's important that lots of light passes through the lid in order to get the heat build-up you need. Make sure to the construct the cold frame around the dimensions of the window or door.