Dont want to bring out an old thread if it has already been discussed but I wante do to ask.....what ever became of your project in making the flange? Where did you buy the metal to do this? I have a similiar project and I am left wondering how to put a much larger outlet pipe on the small flange.
Rotary files, drill bits, piece of angle iron from the hardware store to cut a chunk of steel out of , Hacksaw, some kind of welder. Thinking out loud here. Might score some round tubing off of a old chair leg or something.
I don't know anybody selling exhaust flanges. A forum member might make you one ?