Easy to print, and quite strong. The hook on top is sized for a 13.0 mm diameter round shaft; scale your print to match the size on your seat.

When printed on its side as shown it can be very strong, especially if you set a high number of perimeters. I printed with PETG using two perimeters and the hooks were strong enough to hold my computer backpack. Next time I'll print with 3 or 4 perimeters. Also use a high infill for strength. PETG favors higher extrusion temperatures to enhance the fusion of layers.

The vehicle in the photos is a 2016 Ford F-150 supercab pickup truck. The posts for the head rests were 13 mm in diameter.

UPDATE July 2017: I added extended versions for thicker seats.