G - Remove the glass and backing from the picture frames. Save one backing piece.
H - Add some wood strips to the frame on the non-hinge side to keep the spring clip from crushing the frame.
I - Use the saved backing piece to mark your plastic sheet. Shown here is standard .02" thick polystyrene sheet. You can also use the more durable ABS and clear PETG, but it takes a little experimentation to get the timing correct. Always buy extra material!