Ian......use a conical hole cutter.
Files can be messy and irregular, and cylindrical hole cutters can leave a ragged edge, but a conical will make a nice neat round hole.
I use them a lot for fitting control buttons in stainless and plastic and they're the kiddie for the job.
put a marker pen dot where you want the centre, put an inch of masking tape over it (to stop the drill skating off across the plastic) then drill a 2 mil pilot hole (even if you get one of the conicals shown below, drilling with a 2 mil bit first will be better).
Then go ahead with the conical cutter.
I've used conical ones before but unless you're at exactly 90 degrees you can get a few tenths of a mil bihgger than the step, and TBH you want the hole a fraction smaller for a tight fit anyway, so maybe stck to an unstepped cutter.
PS check behind first.......DAMHIK