I have used flexible aircon duct toroids on all my Tesla coils including
a museum one. Also including one where spark length is 3 times the
secondary winding length. It's great to pull off the Mori clips and
concertina it back to nothing.
I have one brushed aluminium toroid and have never used it.
I don't try to calculate size any more. Bigger is better. I have used
all sorts of bizarre types including a Red Alert reproduction multi
level one.
On the other hand all my home coils have a rotating rod/breakout point.
So if they don't affect performance, are OK in museum and public
performance and are in use on TC's of all sizes then it's just a vanity
thing to use the brushed Al one at great expense.
Just my 2 cents (Australian) worth (and likely to be controversial).
Peter
tesladownunder.com