I've been using this phone mount for my Stadia controller for a while now and I can say that it fits perfectly, hasn't scratched my controller and works very well. There are two small issues with it which iv found hard to correct. 1) the phone position doesn't tilt up enough for me, to get the balance just right for me the phone is fairly low over the controller, it then doesn't tilt up enough for a good view of the screen. This could be adjusted on future models but it seems the position of the ball joint is just not quite right for me. This is a subjective thing and more me than general. 2) the grip for the phone is so tight that it presses the volume and power buttons on my Pixel 4, if I move it lower it activates the Google assistant via the squeeze function. The only way I have found to combat this is to put a case on the phone to spread out the pressure. Again not the end of the world but it took a little thinking to get right. At the very least your phone will never fall from the cradle!
All in all this is a very good product that allows gaming on the go.. exactly what Stadia is built for. These little niggles are small issues in comparison to what you get. I'd buy one again if needed.