Why not just lay a thin piece of hardboard clamped to the steel part, and use a piloted laminate trimmer (no bearing) drill a hole, route out the shape of the cavity, the steel part will template an exact copy in the thin hardboard, then clamp the hardboard to your actual part and use a top bearing bit to exactly copy the cavity onto your work piece? As stated youll have to hand work the sharp corners. If you need more thickness for your top bearing bit to ride on (shallow pocket in your final piece) or you want to accurately test the fit, use the hardboard template to route the pocket in a piece of 3/4" ply, test the fit, adjust by adding tape or opening up the pocket with a file, and when your happy route on the final part.