[vicinc]

I cut mine with a sharp surgical blade which I heated first, cut trough like cutting cheese.
Mark it first with a marker than got tape on the outside of the marking, than cut slowly from outside to inside. In small steps, first just lining the cut, than going slowly deeper and deeper until I got on the other side. Marking is the most important bit I think.
then I polished it with the polishing bit of a dremel. not to bad. I also cut a piece of black rubber (black PE) to cover the hole when not in use. not a bad job at all.