Very carefully
Try using a Dremel or similar tool. Also at Home Depot there are usually 2 different kinds of blank plates . The standard cheap(breaks before you get it home)$1 version and the super deluxe(unbreakable)$2 version. Try the unbreakable version as it will be less brittle. Use a NEW bit at the highest speed your tool has. Go slowly! It should cut through "like butta"
Hope this helps. Good luck!