Review on High-Quality Car Headrest Tablet Holder - Adjustable Arm, Sturdy Aluminum Mount for iPad 7, Galaxy Tab, Fire Tablets | Mount-It! Premium (MI-7310) by Daniel Postle

This product is 100% reliable. The ball joints they use to connect the arms are actually ribbed so they don't just stick to a smooth, round surface when tightened. I was concerned there wasn't rubber between the metal half that presses against the headrest seatpost, but they ended up biting really well. If you think you can spend less and add a spring between the seatposts, don't. I did this and it broke, I had to use plastic tape to fix it and the springs are good but you can't control how tight they are.