I have had a couple JetCat mounts get loose over time - a HotSpot mount and a standard mount as included w/ P120. Rather than use sandpaper, I have cut some 1" strips of thin (.025"-.05" or so) neoprene rubber sheet to take up the slack. It does not move around when tightening up the mounts and doesn't scratch up the case or mount (sorry Matt!).

I have found the sheet neoprene material at an Ace hardware store, sold as gasket material...auto parts stores might have some as well.

Barry