Cutting the back spinner mount plate

[Charles in SC]

I am at the point of doing the prop and spinner. I have a Hartzel constant speed so I have to cut a big area out of the back plate to match the doubler. My question is, what are some good ways of doing this? I know I can grind and file or use a router which I do not like but would rather find a better way if there is one. Thanks in advance!