Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4915994.
Thigniverse is going downhill......and sucks to more every day, let's all migrate to prusaprinters.org instead!
You can find this model from there as well: https://www.prusaprinters.org/prints/79554-xbox-one-controller-phone-holder-battery-cover
I had printed and used the original mount and hthen got this idea when I saw a printable battery cover for the controller. I just pretty much just mashed the two models together and got this as a result (had to add some structural support though). It's very hard to believe but it's actually more sturdy than the original mount, try it and be surprised!
All credits of the original cover I used as a base for this go to d-man5005 whose original design you'll find here: https://www.thingiverse.com/thing:4093750
Compatible and uses parts from this modular mounting system: https://www.thingiverse.com/thing:2194278
The modular mounting system parts used are as follows: 1x Mount Phone Clamp A v6 1x Mount Phone Clamp B with Tab v6 1x Mount Phone Clamp Bolt v1 1x Mount Phone Clamp Knob 1x Knob_v5 1x Bolt M5x25 v1 1x M5_Nut
Of course you also need the file I made: Xbox one controller phone holder battery cover
Phone clamp assembly video: https://www.youtube.com/watch?v=k6nm0oOSpfQ
I highly recommend using metal nuts and bolts in place of the printed ones! I couldn't source a M5x65 bolt myself so I made one from threaded rod (just glued a nut to one end)
You also might need to print the Hex Shaft which is used to tighten the phone clamp knob's cover in place.
I uploaded the needed parts here but you could also make a custom config if need be! Also supplied the .f3d and .step files of the cover for easy modifying!