I have a tray in my 2004 Jeep Grand Cherokee that makes a great place for my HTC One M8 to rest while driving around listening to Pandora radio. The problem has been that the tray is plastic and slippy (a word commonly used in Pittsburgh), and most left or right turns would cause the smart-phone to fly away. I can't fault Jeep; the car was built 4 years before the iPhone was invented, although the true usefulness of the tray is a mystery to me.
I knew I needed to make the tray's surface "stickier", and I recently found the perfect solution - spray-on rubber coating. Three coats and the problem's solved.