I liked the concept by beer92266 (Thingiverse) because it didn't require supports and would result in strong parts with little load going across layer bonds. But I wanted to make one that would fit snugger to my 13mm diameter posts. I also thought I might as well make a slightly different shape for the other hook as well.
No need to print very solid unless you want to hang bags of rocks from it or something. For the heck of it, I made a single hook set printed a crazy 7 walls thick with petg and was able to hang a 40# bag off it without any sign of stress. I would be more worried about the damage to the upholstery than the hook.
I made some different sizes of each of the hooks, so hopefully you can find one that fits for you. The headrest hook comes in 14 and 18mm diameter hook sizes with 54 and 80mm length options. (measure from front of post to rear of seat)
I made the 14mm dia x 54mm headrest hook and the larger seatback hook part.