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.
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.
In order to retain heat inside the cold frame, it's necessary to add insulation; you can use insulation boards used to face houses. Take the measurements of the interior walls in the cold frame box and transfer those to the insulation boards. The boards cut easily with a sharp pocket knife or box cutter. Install the insulation to the inside walls of the cold frame. Use construction adhesive to attach them. Add 1-1/4" screws to each piece to keep the insulation in place as the adhesive sets up.
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.
Pre-drill the holes the lid's frame because the wood is narrow and will split easily. Then use 3" galvanized screws to screw the lumber together; the wooden frame for the lid is complete. Lay the Plexiglas on top of the frame and square up the edges of the glass with frame to make sure they'll fit together. Remove the Plexiglas and run a line of construction adhesive around the top edge of the frame. Then place the Plexiglas back on top of the frame. Using a Plexiglas drill bit, drill holes every 4" to 6" around the frame and secure with screws. The adhesive and screws together will make the lid more water- and airtight.
Use utility hinges to secure the lid to the top of the cold frame. You could also use window or gate hinges. To attach the hinges, center the hinge joint between the lid and top of the frame box at one end of the cold frame and mark where to pre-drill. Repeat step at the other end. Then pre-drill the holes for the screws; put the hinges in place and attach them with 1-1/4" screws. Next, measure in 22" from the center of the outside hinges at both ends of the frame box. Mark and pre-drill the holes and attach the last two hinges with screws. There should be a total of four hinges securing the lid onto the cold frame.