I thought that the tapered aluminium tube was a bit marginal. Maybe it's OK, maybe it isn't, but in view of your comment, Jami, that you expected to sail in strong conditions, I came down against it. It won't be a case of short term catastrophe, with failure in the first big blow, but more a case of "what is the service life going to be?", remembering that aluminium alloy does not have a fatigue limit, a fixed level of stress below which fatigue failure does not occur (unlike steel, for example). The higher the stresses, the shorter the lifespan, and the lower the stresses the longer the lifespan, but every aluminium mast is going to fail, eventually. This isn't really a problem, we have to remember that every aircraft flies on the basis that its components have a safe lifespan, after which they need to be replaced. We just have to ensure that the service life is as long as we need it to be.
But notwithstanding the above, in view of your difficulty in finding anything else, it's looking like it has to be the tapered aluminium tube. So how to reinforce that vital area just above and below the deck? We've eliminated a matching sleeve of aluminium tube as being unavailable. I think I'd put on unidirectional glass/epoxy, with the fibres vertical. A cloth 1 metre wide will go around twice. I'd put on one piece 1 metre long, so 500mm above and below deck, followed by two more pieces, increasing in length, a 2 metre piece and then a 2.5 or 3 metre piece, so that it extends to the heel and then to the same distance above deck.