Position window on lawn, then raise one end up to the angle desired using a piece of wood.
Surround window with wood boards to determine board lengths required, taking note that the front-to-back length needs to be shorter than the actual window depth, according to your chosen angle.
Cut boards to create the planter box.
Cut 4 2x4 corner supports to the height of your planter box.
Assemble box sides by placing side plank/s down, then attaching two 2x4s on each end. Do this for both sides.
Position the remaining boards against the assembled boards, creating a rectangle. Assemble loose boards to 2x4 supports with screws.
Protect inside of planter box with Tung Oil to preserve wood if desired.
Place planter box into garden.
Position window on top of planter box. Attach two hinges to the fence side of the window frame.
Raise the back of the window to the angle desired, supporting it with a piece of wood, then attach the hinges to the fence.
Attach a cup hook to the front of the window frame, then while open, attach another cup hook directly above it on the fence. Connect both hooks with the chain.
Fill planter box with soil, then enjoy your new cold frame!