I don't see what the problem is.
Why don't you just use the 2" adapter that came with the scope and a 2" Camera Adapter, that is the maximum width opening that you can buy as standard, my Tak has 2.7" but the 2" adapter is fine for the 350D, I get no vignetting with the wide Tak T-Ring.
or a standard 2" TeleVue for that matter.
Cheers
JohnG