Headrest hook

Print each part flat.
Glue the "hook" part to the "bottom" part.
Cut a suitable length of strap (e.g. from an old rucksack).
Loop the strap through the top and bottom parts.
Sew the strap to capture the parts.
Lift the headrest from the seat.
Mount the hook assembly.
Mount the headrest.
Enjoy.

UPDATE: It seems like one of the files I uploaded was invalid... It should be ok now.