I used a thin cut-off wheel in an air die grinder.
L.Adamson --- RV6A
Drill a pilot hole, enlarge it with a step drill to a hole size that allows an Adel nilbbler ( http://www.averytools.com/pc-248-41-adel-hand-nibbling--tool.aspx
This nibbler give you real control of the material to be removed. Sneak up to your cut line with a combination of files and 3M Scotchbrite wheels http://www.averytools.com/pc-351-43-scotch-brite-cutting---polishing--wheel--kit.aspx I used the 936 mandrel with a 3719 wheel.
Whizz wheels or cutting wheels can get away from you IMHO.