I have remixed two thingiverse designs together as I was after a car CD mount for my OnePlus 3.

This version 1 has the CD mount prongs down the bottom and the phone will sit above the CD slot. No need to print with supports

As per Truckerjames creation, this works with the official OP3 case

I know its not perfect, but feel free to modify and remix it for yourself, Thanks

Version 2:
http://www.thingiverse.com/thing:1745951