Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:5427544.
Designed to 1) fit most headrest post diameters, 2) have the cut out angle on the anchoring plate positioned relative to the head rest post (not the plate itself) so it accommodates for the tilt when the holder is used with some weight, 3) be a single unit so that it be stored/retrieved better and to avoid stress or tension on any hinge joints, 4) have a wider contact surface on the supporting hook to have less "crunch" on handbag or pocketbook straps, and 5) have a uniform width on the holder to avoid stress points around the bends. Corners were filleted so indentation marks weren't made on the car seat after a longer ride with heavier objects.
At this thickness, it can support at least 15 lbs, but not sure for how many car trips or for how long of a drive This was tested on and seats well on headrests from 3 different car makers, please let me know if it doesn't fit yours