The ultimate headrest hook.  The headrest clip fits around a headrest post completely and will not fall off while driving or being knocked around.  It attaches easily without having to remove the headrest.  The chain will conform to any seat back. 

The chain and hook print in place, you shouldn't need to change the orientation.  I've found it prints very well at 0.28 mm.  After removing it from the plate force it to bend, it will crack a bit as the layers release and after that has a smooth action.  Paint on a support under the prong at the end which will be used to connect to the headrest clip.  Do not allow supports anywhere else as they will interfere.

I've printed this successfully in ABS and PETG, both of which will hold up to the hot temperatures of a car in the summer.