Thanks for the link Rod
.
Your insight and this site has changed my approach!
Now I'm toying with the idea of hybridising Albert's design!
Albert's design seems a little light in support for the alt bearings. My idea is to incorporate the Sonotube section containing the mirror and float into where Albert has his "fibreglass" tube, and add some more bracing for the alt bearings. I realize this makes the thing heavier, but Albert's mirror box design I'm otherwise following.
I've ditched the Sonotube cage, though I'm yet to finalize my cage design.
Thanks again.