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The rough-cut flange was bolted to a master flange used as a template and cut to match on an overhead arm router (a regular router used in a table would work, as well). I used an ordinary wood-cutting carbide-tipped 1-inch flush-trim pattern bit with a ball-bearing follower. The outside was cut in the same manner. Hand bar soap was used as a lubricant to produce a smooth cut, so precise that the master and cut flange appeared to be a single unit.