I used a dremel cutting wheel and followed a straight edge working from the top of the ring out...(toward the fuse end of the ring).
It's easier than trying a knife to get through the plastic and it's not too difficult to control.

Just go slowly and wear eye protection, that cutting wheel will break at least 2 or 3 times before you have them all done.

Another way is to use an xacto saw fitted to a large handle.