Here's a creative idea: Build a frame out of 2" x 2"s larger than the opening and install door casing over them flush with the outside of the frame, overhanging the inside of the frame. Then install translucent plastic panels resting on the ledge created by the casing, you may need a "T" molding in the center for support. It will look like a florescent light fixture. You could even add light to it by using LED tape lighting, just apply it to the stairs wood panel.