I used a 10mm outer diameter, 8mm inner diameter CF tube from rcfoam.com
To be honest thought if I had it to do again I'd go with a much larger diameter, thinner walled tail-boom. I think that'd resist twisting loads better yet still be around the same weight. Instead I went with a small diameter, thick walled boom. She seems strong enough for regular flying but I'm concerned about long-term durability & also hard landing survivability. Again I think larger would help with that. A tapered tail-boom would be real nice too. I just didn't have the money to spend on something fancy.
Maybe poke around some more & see what other similar ships are using.