This is a spring clamp that uses the spring of a standard clothespin. It is just like a regular clothespin but with bigger jaws and small pivoting surfaces for better grip.

There are three STL models, with varying jaw length. You will need two of them (of the same size, obviously).

The pivoting surface is printed in-place, you may need to do a bit of force to free it after printing. Just give it a big twist.

UPDATE 2022-02-20: I've added two notches for the spring, so that it should fit long and short springs. I've also added a notch to the top of the arms so that you can fit a rubber band if your spring is not strong enough to hold your piece.

Designed with OnShape, model available here.