A spiral flue bit, looks like a drill bit, but if you look close the flutes kinda flare out because it's designed to cut sidewards as well as down. You should have got one in a typical dremel kit.
Otherwise any kinda routing/milling bit that can cut sideways. Just go shallow and expect to do several laps to go right through and you'll have no problems
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Otherwise any kinda routing/milling bit that can cut sideways. Just go shallow and expect to do several laps to go right through and you'll have no problems