My significant other always buys enamel pins while traveling but those cheap holders/pin backs never hold and she always lose them. 
So she asked me if I could print something for her.
Ive quickly sketched up a pin holder and printed her a handful of these, then my friends asked me if I could also print them some aswell so I thought I will share my quick sketchup so that others could print them too.

The material you print does not matter Ive print them with PLA. Altho depending on the pin size you might need to scale the model by a couple percent. Ive printed them so that when in use you could only remove them with pliers.
Because they are so small Ive wouldnt bother with infill.