This adds a corner to your door that allows your average cat to go trough it. By sliding the corner piece into the holder you can fully close the door when necessary.

I made this for a 39mm thick hollow door. If your door is solid: adding some extra screws in the middle might help with sagging of the holder. I intend to paint this when I repaint the entire door, should improve the look of it, but no idea how well that will work.

You do need to cut the corer of your door of course. Don't over tighten the screws or you will break the holder. I've added holes at the bottom of both pieces for 3x2mm magnets; use some glue to secure the magnets when necessary. The magnets should give a nice closing feel, but the friction alone might make this totally unnecessary.