Go get some gasket material at the auto
parts store and make your own. You can set it over the flange and tap around the edges with a ball pein hammer to score the pattern, or make a pattern out of a piece of paper rubbing a crayon over it to mark the flange.
Get a chunk of aluminum or something fairly flat, wrap some sandpaper around it and go at the flanges to make sure they're flat before putting it all together.