So it is a hole for a mast, not a flag pole. I think spinning the shape would be difficult because you will need a return on the shape. On the Peterborough sleeves they have the main sleeve which lines the hole and the decorative bit you see on the deck is turned back from that. As spinning starts with a flat disc which is then forced over a turned form, I can't see how that would work. Spinning is great fun though. It gets a bit hairy on a larger scale and you must keep the metal well annealed.
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I hope to be proved wrong!