Key Facts:

• Should work for most modern-day smart phones
• Made from only two 3D-printed parts and one rubber band
• Use stick-on foam strips to stop the mount from pressing your phones volume/power buttons. May not be an issue if your phone is in a case.
• Only contains files for the OG Xbox One Controller and the OG Xbox One Bluetooth Controller. I.e. I don't have any models that work with the Xbox Series controllers at the moment.

Update (20th Nov 2019):

What timing, a week or so after uploading this design, Google announces their Claw phone mount for their new Stadia Controllers.

My gripe with existing controller phone mounts:

Most of the controller phone mounts available today position your phone above the controller at an angle. While this is fine if you're sitting at a table, in every other orientation it's terrible. On a plane, laying flat on the couch, etc, are all uncomfortable. Also the controller with your phone installed is unstable, leaving some to add kickstands to their mounts to keep the controller + phone installed sitting stable when you put it down -_-

This design is really comfortable almost all of the time. When laying flat on the couch, it's a little front heavy, however if you we're to design a portable battery mount to hang off the back of the controller, the counter-weight should solve this problem.

Regarding the pictures, the black controller is one of the original Xbox One controllers (no bluetooth). The pictures show my original version where the phone was a bit higher than necessary. I've subsequently brought the phone closer to the controller in the STL and Fusion360 files uploaded. The Fusion 360 file is there if you want to adjust the offset.

The white controller is the new Bluetooth enabled Xbox One controller. I've left room to plug into the headphone jack and the micro USB port.

You can remove the battery cover while the phone mount is attached for both controllers.