I needed a cold frame to start my seedlings this spring and I had torn down a wall in my shop, reclaimed most of the wood, several 2x4's, some plywood full of nails, and some cedar shakes. I also had some pexiglass that had also been reclaimed from some advertising signs from a friends workplace. The only pieces I purchased were 3 sets of dollar store hinges, and some para cord to hold the windows open, all under 10 bucks!!! I sized everything to the pexiglass sheets, drawing dimensions are very ball park and not to scale!!!
Step 1: Window Frames
I started by ripping a couple of the cleanest 2x4's about an inch wide, then I set the table saw to cut a grove for the pexiglass to sit in. The corners are just butt joints that are glued and screwed. Make sure the frames are square or will cause problems later in the build.
Step 2: Building the Box
Start by cutting the end pieces from some plywood or chip board, the length of the angled piece is the same length as the window frames, I then sized the three windows together and made that the width of the box, I cut 4 2x4's to that length and screwed it all together making sure it was square. Make a few trial fittings of the windows to make sure everything is going to work out.
Step 3: Finishing the Box
Measure the back of the box and cut plywood to fit, I used screws to fasten the plywood back and made box more rigid. I cut and used the cedar shakes on the front for a little detail, but plywood will work just fine if that is all you have.
Step 4: Hinges and Upper Frame
Attach the hinges and screw together, make sure there is a small gap so the windows move freely up and down. I cut two vertical posts about 44 inches high and cut another piece for a horizontal bar for the para cord window holding device.
Step 5: Window Holding Device
Drill holes in the lower center or the window frames (as shown) for para cord to thread into and make a little pull loop, tie a knot on the back side, then run a length of cord up to a hole in the horizontal bar then over to and through another hole in the vertical post. I made some wooden cleats as shown, a little cutting and chiseling, and screw with 2 screws each cleat. Just lift the window with the loop and tie the cord off at the cleat so you can water, if sun shining I leave the center window open so as not to get over heated. The frame is set facing south for the most sunlight, I used this cold frame to start tomatoes, cucumber, squash, pepper, broccoli, lettuce, sunflowers and spinach. You could also use it right in the garden over rows on the cold nights. It worked really well for me, good luck!