We wanted to be able to block the stairs from a visiting puppy, but still allow our cats to get down there, so I made these brackets in SketchUp and printed them. They mount to standard door moulding and hold a piece of Plexiglas. The felines can get under it, the puppy can't. Yes, the top brackets are symmetrical, you could just print one twice. One of the bottom brackets is truncated to avoid a piece of wood, if you don't want that you can edit one of the other shapes or flip the opposite bracket (which I "truncated" with a bandsaw). The gap for the plexi is 6.3 mm, which is surprisingly under 1/4" but the plexi fits fine.