Isolate Face/head With Arkit

Hi,

I'm just starting to mess around with ARKit in Unity and I'm having a bit of trouble figuring out the appropriate way to accomplish isolating the face during tracking. Essentially what I am trying to do is once I have the bounds of the tracked head, create a mask around it so only the tracked head is visible on screen, basically creating a floating head. I've tried messing around a bit with culling/occlusion masks but I don't think this is the proper way to go about it. Any help or point in the right direction would be very much appreciated.

Thanks!