Rectangle to Round Transition

This is a crude example of how my company will fabricate a square or rectangle to round transition. Each flat side is simply a triangular piece of plate. The corners are press broke into conical sections, the extra material on each side is to accomodate starting the piece in the brake. After forming and a rough fit-up the extra material is sheared off. The parts are then assembled and welded. The largest one I've seen done used 5/8" plate, and transitioned from a 120" x 144" rectangular opening to a 96" round duct.