Lots of ways to do it. In hindsight, drilling out would have been a good idea and saved some time. Keeping your dremel/RZ dust free while you work is another good idea...trust me.

The hardest part was getting the backs concave so they sit over the outlet flush.
Your tools should work well with a soft stone, just be aware of too much vibration for cracking. And take your time. If you enjoy putzing, it will be a rewarding, but time consuming project.
If you don't count the RZ we ruined, we figure each cover was about a buck.