Measure twice, cut once. Maybe I should have measured that second time.
I built this from memory, since I was sure that I had an accurate frame of reference by way of a socket that I'd used. A no-name spark plug socket (for the internal square drive and external hex) trimmed on the lathe to accept a brand new 6-point Kobalt socket. TIG welded together, then turned on the lathe mostly to high the fugly welds.

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And the failure. The tool is sitting on the tensioner pulley. It's entirely too long and utterly useless for the intended purpose. The silver nut just under the tool is where it's supposed to fit.
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Time to start over. V2 will either be a plain socket with flats ground onto the side, or something with the mangled flare nut wrench that I actually used.