I'm guessing that you plan to take them in the cabin versus checking them? If so, I would recommend going with soft-sided carriers, because they squish down to the airline size requirements and then re-expand for the cat size requirements. Something like this looks like it is exactly 12 x 8 x 17. Of course, the carrier and cat have to weigh less than 20 lbs, which can be a problem with Maine Coons.