Buy yourself a bit of 10mm steel plate ( Mascot Steel Fabrications if in Sydney are good for this )
Mark pattern off gasket
Drill with a good quality holesaw in a drill press
Cut to shape with a cutoff wheel in a small angle grinder
Buy a length of tube and a selection of bends from your local exhaust shop
Mig the tubes to the inside of the flange and braze on the outside to get a wide fillet, clean up the inside with a dremmel / die grinder.