I don’t believe putting plexi-glass on a roof hole would be a good thing, unless it were very small. I would be more inclined to make your ventilation areas a little larger it you wanted more light, not on the very top where the sun bears down. I even have to put a Styrofoam heat shield over my wood roof to keep the heat down, as my boxes are in full sun. Gin is definitely correct in that the correct box and location is what will attract the birds. Good luck.
