I made my own 4 into 4 in-cowl exhaust using Aircraft Spruce 1 /1/4" dia 321 stainless steel bends and 1/4" s/s flanges. I'm no welder.
I iteratively taped them up , got a friendly local TIG welder to tack some of the pieces, etc, etc until the whole thing was tacked and fitted. Then he completed all the welds. Beautiful job!
They fit perfectly too - if you know what I mean!!

Regards
Steve