I recently printed this paper clip from a model on Makerworld:

The holding power comes from a separate cylinder that keeps the clip under enough tension to stop the papers pushed into it from falling out. The original was intended to be fastened to a wall with a sticky pad but I used what is called a “negative part” to create a hole in the back. That means it fits over the screw I already had in the wall.

I put it up on Friday and the calendar I mounted with it is holding firm so far, so I’m confident that it is a functional design.