Use an angle grinder or reciprocating saw on a drawn circle. The saw would be easier. Even a jigsaw with a metal blade would probably work.
Grinding out with the bit is going to suck some ass.
Neil -- "Easy Bake Master"
NB Bandera [gone to heaven...conveniently located in Long Island, NY]
Rebel 23, WSM A, WSM B, UDS, Easy Bake Oven
Pork Pistols BBQ
MOINK...It's what's for linner.