Traced edge of opening on paper, cut it out & transferred it to patch material. Used a metal cutoff wheel (13,250 RPM) on hand held DeWalt 4.5" 10K RPM grinder with Backing Flange to cut patch a little big. Ground to size and beveled edges on pedestal grinder. Also beveled hole edges with hand grinder. Positioned with magnets. Tried using attachment for cupped grinding wheel insteal of backing flange. Exploded several cutting discs resulting in pieces flying through the air. Life is to short. Paid $5.75 for my generic backing flange. DeWalt sells them for $2.97 but are hard to find in local stores.