We have many holder that can hold a CP Filter to your iPhone or any other smartphones.
For Example, this quick clip.
How it can use on an iPhone or a smartphone?
Here you are:
Very easy, just attach it to the iPhone and in front of the main camera.
To make it a little more stable, just turn it to be side to the iPhone or smartphone.
then you can use the CPL.
Of course, you need to turn to find the best polarizing effect.
Here is a video to show you: (the video is with our another clip.)
Click here to learn more: DCKina