Mark the hole out VERY clearly (white out)
Use a 4" grinder with 1mm cutting discs to cut the shape out as best you can, leaving at least 10mm to go.
Use a grinding disk to get the rough shape, thena flap disk to neaten the hole up. Take your time and you will get it right.
I've never had an issue using a grinder on guards, although have used an air nibbler & jigsaw.
Mate got a bit carrier away with the nibbler - not the desired result, so I will never use one of them (partly because of my lack of experience with nibblers)
Cheers.
Dave

