I'm going to be soldering a lot of 12 mm momentary buttons onto cables soon and so it was worth making a jig. This isn't perfect, but it's a good start for holding the cable and separating the ends.
Probably best to print this in PLA.
v1.1: added eject hole under button for when tolerances are too snug (at suggestion of Jake at Makers Making Change).
v1.2: added tiny tolerances around button as people were complaining about it not fitting.
Mind officially blown by a recent workshop participant who used the jig with the button upside down. This works much better!
You're probably going to burn this a bit with a soldering iron. PLA smells a lot better than ABS when this happens.