Good thinking Stefan. I can think of another way to do it but that's probably the best way.
However as you did say you'd finished this on a lathe, I'd have included some steps in the mould and faced them on the lathe also, to mould-in accurate shoulders in the carbon fibre tube for alignment purposes. And sure they'll probably need a skim in the lathe afterwards to make sure they're true to 0.01mm.
But personally I'd probably have obtained (or cast) a lump of duralumin alloy big enough and hogged it out. An 8kg sledgehammer will punch holes in engine blocks if you swing it hard, and I used to have a mate with a furnace who melted them and cast the blanks for me for a 10" worm and clutch. A couple of engine blocks = a LOT of alloy
and free.