I love my new Tesla Model X, but quickly realized there was no place to hang my suit coat or dry cleaning so I decided to make my own. I used a Type a Machines Series 1 Pro printer and Cura V1.5. I used 3D infill at 3mm separation (Cura uses space between infill instead of percent so it is nearly solid). The material I used is Proto Pasta Carbon Fiber PLA and it is plenty strong. PLA may work, but you will have to experiment. I am pretty sure ABS will work as well.
Note: I used small self adhesive rubber feet on the back of the hook to limit scratching of the seat back as well as hold it in place while driving: https://www.amazon.com/VOVOV-Self-adhesive-Rubber-Bumpons-diameter/dp/B016GWHSBE/ref=sr_1_5?ie=UTF8&qid=1504108807&sr=8-5&keywords=rubber+feet+for+electronics