S-biners are common useful tools for all kinds of light-weight, small connection needs.  They're good for things like keychains and other random attachment needs, and perhaps you'll find a good use for yourself. 

I recommend printing in PETG for the flexibility and plasticity.  PLA is likely too brittle for the clip gate to work well. 

The “small” version is about as small as you can go with a .4mm nozzle.  The “medium” is a good standard size.  Scale it up if you like for bigger applications - should still work. 

After printing, let the print cool, then apply a light twisting motion to move the gate to the inside of the clip on each end.  Flexing the gate too far into the clip may plastically deform the gate and make it not spring all the way back into the closed position.