From the sounds of things maybe start with a part of the ring (near the bottom) and experiment differing ways of cutting into it. Right now first idea for an easy job is to heat up a blade and cut through the plastic. I'm thinking a box cutter would work (one with an extendable blade of some length) and just heating it using a soldering iron or something. Careful with heat of course. (: oh. An of course, careful with the blades!

Once cut off, you could sand down the underside.

With the drilling you should be able to pick up one of those hand twist drills from a hobby store. Just make sure the kit comes with the right sized drillbit.

Let us know how you go!!