Update 12/8/19: Due to popular demand, I've added the files from the modular mounting system, so this should now be a one-stop-shop for all the files needed to get this working. However, the phone clamp has been updated since I created this design, but the controller mount should still interface with it (confirmed to fit and work by HoaxXBL's make: https://www.thingiverse.com/make:740947 Thanks for posting!).
A clip to mount your smartphone or tablet onto an Xbox One S controller. This is remixed from the XBox One Controller Tablet Mount here on Thingiverse.
I modified it to be compatible with the Modular Mounting System since I really like their phone mount design.
I will note that I was unable to get all the buttons on the controller working when paired with my android phone. I ended up using my wired Xbox 360 controller instead, and I remixed a mount for that as well.
Thanks to Running_Banana for summarizing the parts you need to print:
1x Xbox One Clip with Modular Mounting System
1x Phone Clamp Up with Tab
1x Phone Clamp Down
1x M5x25 Bolt
1x M5x65 Bolt
2x M5 Nut
1x M5 Knob Short
1x M5 Knob for Phone
You get all of the nuts, bolts, and phone holder from https://www.thingiverse.com/thing:2194278
You can view the design files here.