Took out my old CPL Lens from my Hero 3 and put it in my Phantom 3 Pro.All i had to do to make the CPL frame sits in the P3P Camera frame is to stick a small double sided tape
on the upper back of the CPL frame. Its actually very snug and tight and easy to remove too...

Have you actually flown it like that?  You'll have to give us an after action review to see how it handles the vibration and movement.  (and video!