I usually make jigs if more than two of anything. Need eighteen of these. Not difficult to set up, bit of rod slit endways and ss plate just sits in the slot. They will slide in the slotted tube tube with a bit projecting to attach a sail to. Commercial nylon slugs are available for normal mast sections, but not of this size. I did think of making them out of ss hooks with a moulded epoxy/graphite slug. Bit of a faff though with the moulds.if you can make some sort of jig to hold the two parts in place (of just stop them moving) and not be where the solder will go that would be ideal. bend a U section of solder around the joint, assemble with flux and heat up.
it does depend on how many you're trying to make though