turbo wastegate seat erosion help

It's only a waste gate flapper valve that opens slightly when the turbo boost gets too high to vent exhaust around the turbine. It doesn't get hammered, just closes with spring pressure when there is low or no boost. It will see extended exhaust gas velocity when WOT and won't have a traditional valve to absorb any of its heat. How hard would it be to cut the valve seat pocket with no arbor (valve) hole to guide cutter? Mill work?