If my memory serves me right, CPF modder Tana worked out a method for boring an A2 Aviator for 18mm cells, but it had to be the later, round-body version, not the 4-flats. Using 2x18350 cells would give you - what - about twice the runtime compared to 2x16340(?). Even if you could get 1x18650 in such a bored body, you would not get the same performance as 2x18350. I think that all of this is in a lengthy MOD discussion thread by Tana, which I don't have the link to. If I got any of this down incorrectly, please feel free to correct me.
I don't know if anyone else can bore an A2 Aviator to 18mm, but if they do, I would love to hear about it. Tana has a stellar reputation for the quality of his (gender assumption) work, and I mean no disrespect, but I'm in the U.S., and would simply prefer to use domestic mail.