[john70t]

Originally Posted by bugstrider

I hope the link works, if not I will remove the post
https://www.reddit.com/r/INEEEEDIT/comments/8ni82h/simple_locking_hinge_to_convert_a_door_corner/?st=JHURLII0&sh=48b42d57

That 'triangle lift hinge thing' probably won't work.
Cats will wonder why the door thing suddenly doesn't work today.
And then they will proceed to claw a hole through it.

There are some nice pet-portals on the market.
The previous owner put in a budget square door opening, which could have a spring door or other on it. Whatever works.