A simple block you park on the top of the door, keeps the door ajar just so much and not more, depending where you put it on the door, closer or further from the hinge. I designed this for my daughter's bedroom so the cat could be assured access back and forth from her room when she was gone, as my autistic grand child would keep closing the door, either trapping my daughter's cat in or out of the room. A problem when the cat needed to get to it's littler box… The setup was completed with a door chain that allowed the door to be secured, again, only so much, while keeping said child from entering the bedroom and providing a safe haven for the cat.
The author marked this model as their own original creation.