My recommendation would be a die grinder or Dremel tool with a small diameter, drum sander. This will give a nice nearly finished edge with no sharp corners that could result in a crack. The notch doesn’t have to be small. It just needs to be small enough that it doesn’t get too close to the next Adjacent screw holes.
Opinions, information, and comments, are my own unless stated otherwise.
You are personally responsible for determining the suitability of any tips, ideas, etc. obtained from any post I have made in this forum.
Formerly of Van's Aircraft Engineering Prototype Shop
FAA/DAR, A&P, EAA Technical Councelor
RV-6A (aka "Junkyard Special ")