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This cold frame will be 6' wide and 3' deep. Use 2x12 lumber on the sides and back and 2x8 lumber on the front side where the access to the frame's interior will be. Measure one 2x12 lumber 72" long for the back, one 2x8 lumber 72" long for the front and two 2x12 lumber 36" long for the sides; cut the boards with a circular saw.
Lay the pieces out to form the cold frame. In order to catch the energy from the sun, the top of the cold frame needs to be angled at least 1" in drop per foot of depth. Since the box is 3' deep, you need at least a 3" drop from the back to the front. On this project, there's a 4" difference between the heights of these boards so you can use that as the measurement to angle the sides. Make a mark for the lowest point, where it lines up with the 2x8 lumber at the front. Now take the level and draw a line from this mark to the upper back corner. Repeat this step on the other side.
Cut along the angled lines on the sides of the cold frame. The straighter the cut, the tighter the fit will be between the lid and the frame. This slope will support the lid of the cold frame.
Use 3" galvanized screws to attach the sides together. Pre-drill the holes before putting in the screws to prevent splits. Add L-brackets to each of the inside corners of the box; put two brackets per corner, one near the top and one near the bottom. This will keep the corners from splitting apart when it's moved.