I started by experimenting with creating a tension mechanism that would lock into place.  I had no idea if this could be done with PLA, I just wanted that satisfactory "click."

The choice to use PLA in this way is not ideal, but I'm from the school of use-what-you-got, and I wanted to get this operational, so I stuck with it and made more versions.  I continued to experiment with different shapes and forms, moving groves and ridges in an attempt to create a clip that would snap shut, but pop open when pushed by a servo.

I also experimented with different methods for the clip to hang.  In this version I was trying to prevent the clip from twisting.