As a test, I took a 2 1/2" hole saw to a piece of 1x4. That gave me a hole that was between the 2 sizes of the pipe ... a little bigger than the "male" side, and a little smaller than the "female" side ... just what I was looking for.
Then I used a smaller hole saw as a sort of a "poor man's router", if you will, and rabbetted one side a little to fit the female side in nice and snug.
I may very well use this for the exhaust side ... why not? I just need to drill a hole in the eventual top and glue or screw it in place.
I can make another one for the intake side.