After finding the center for the exhaust tube (red arrow), I drilled a hole for the air fitting (yellow arrow) to be inserted and welded into place. This can also be done with a torch if you like. This is tricky since the pipe has a bend making this a complex geometry. But I wanted the center tube centered which I thought would keep heat even around it. The air fitting (yellow arrow) is inserted in the plumbing cap (green arrow) to show the good fit.

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