Apple specs say that an iPhone X is 2.79 inches or 70.9mm wide. (I actually measured it to be 70.97 mm and 2.794 inches with no case. (Excluding buttons)Right, we are using "narrow" backwards. That said, I don't use cases on my phones, so I need a snugger fit.
I measured it with the leather case on it using a digital caliper and it comes out to 2.942 inches or 74.67mm.
According to my calculations that means that you would need to add 1.85mm or .074” of padding to each of the rails to approximate the same snug fit on a bare phone as I am experiencing with Apple’s leather case. I certainly would not want it any snugger. It is a pretty tight fit. I think there’s a little more going on with the case then just the width. It seems that the increase in thickness of the phone is also responsible for the snug fit. There seems to be quite a bit of pressure on the fins that hold the phone against the back rest.
I imagine a strip of adhesive felt from Home Depot will do the trick.
I would advise wrapping the felt to the inside of those fins as well.