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I think you are looking for a GREENLEE punch used most commonly by the electrical trade for adding knockouts in panels. You can not use a hole saw as the center is already gone from smaller gauge if I guess right. Go to any electrical supply house or maybe Ace hardware (they stock them here in Phx. ) but they are a little spendy.
I have used a good carbide bur on a die grinder with good results. I tape the area with 2 or 3 layers of duct tape just incase it skids on me. Then hang vac. hose near work and brace hand against work to control better. Go slow and good luck!
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