I agree with Bill and it is possible, but the trick is trying to keep
the channel some what air tight, so that you get the full vacuum affect
from the fan. I just put in a 30" last weekend. Pretty fun stuff and
not that difficult at all.
I ran into an issue where one of the joists that my fan was going to
rest on, had a vertical support to the roof on it, so I had to shift my
fan about 1.5" over the whole. No bad at all. It works well.