Here is my addition to the Modular Mounting System

Here is a really good remix that fixes issues like sliding and ability to reach the sync button without removing it. id recommend downloading that one as i wont be updating this one
https://www.thingiverse.com/thing:3189418

A Xbox one S Controller mount ( the one with Bluetooth for phone connectivity)

Fits snug as a glove!
Very easy to attack and clip to the controller and also very easy to remove!
Does not wobble about while attached to the controller stays where you leave it.
Has a hole for a charging cable for the optional battery pack that does not effect the stock battery cover.

EDIT. I have expanded the hole for the Charging port to allow room for thicker cables and give some variance in the charging port position not all battery packs have the same charging port layout.