Thanks Alex for your kind comments
And I too think I should do something interesting with the tube colour!
The alt discs actually attach to the tube rings very securely and rigidly as there is a flat mounting pad on the rings. Once screwed on tight there is far less flex in these parts than in other areas of the mount (which only has minimal flex). I will need to ensure they are square though. I'll give it a try as is, I can always fashion a support bar that goes between the discs and sits under or over the tube (or both). I'm not sure why they didn't go on square today, when doing a dry run early on they sat very square, so maybe I just need to be more careful doing up the screws.
EDIT: wrong name sorry