Ok... You can either...
1.) Just jig/dremel it out, and then file/sand it with either a file/rasp/sandpaper until it's nice and smooth.
2.) For fans, get a hole saw, perfect cut every time. They make them up and past 120mm.
3.) Use a guide for a dremel, like thier mini router attachment. Then setup a wood plank to run the dremel against, this will give you a near perfect cut.
I'm sure there are other ways, but those are the ones I know of.
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