Header Fabrication

One thing that on serious note will actually help you vs our sarcastic wit is to fabricate a “horn “ that matches the rectangular size you are trying to fit to. Then as you heat and beat pipe to fit you can have a template if you will, to test fit while still hot. And the Tom Hoover quote is as relevant today as it was back then. Straight up fact! You think fitting to the rectangular port sucks wait till you get in some tantric yoga positions with a mirror to weld the mitre.