What Die Grinder bit to use? Need to enlarge antenna hole in fender

The above answers make sense, but I went a completely different direction on my Barracuda. I filled the hole and bought an amplified "hidden" antenna. Similar to this one.

RetroSound HPA-1 antenna
I would have preferred they left it alone as a radio delete car as well.

After all these and the Big Brother website FABO replies, I think I’m going to use a 1/4-3/8 ish carbide burr and take light passes with it until the desired hole size is formed. I will drill the two small locating slots first with a drill bit. I will decide which size burr to use once I can see them and hold them in hot little hands. The Larger 3/8 diameter is probably going to win that decision.

All of this after taping many layers and etching a fine line in the paint with a sharp edge to prevent chipping beyond the cut of the carbide burr.
‘’thanks for replies all