Here is an filter ring adapter that lets you get more creative with your smartphone camera. The SmallRig 67mm Filter Ring Adapter is made for phone cages or cases with a M Mount. It does not block your phones cameras and lets you use various filters depending on what you are shooting.
The 3839 adapter ring locks onto your phone with a simple clockwise rotation. You can use it with ND, UV, GND, and other 67mm magnetic filters. You can attach and detach this adapter in seconds.[where to get it]