There are two versions: one uses zip ties to keep it on the controller, the other one a rubber band. The zip ties I used are 7,5mm wide and can be undone.
The grip is also a little oversized so you will have to put some thin foam in the inside (like on the picture). One grip uses 2 M4 thumbscrews that have to be around 11mm long (the threads).
The top of the 25mm webbing / belt must be melted to form a wedge in the inside of the slot.

UPDATE.
I modified the grip (with a dremel) so that the battery cover doesn't get removed with the grip (added so images).

No

No

0.2

50

I used wall tickness of 4.