Ah sorry didn't realise you could easily have two carry on. Does it have to be a backpack? I'm guessing this will be an occasionally to rarely used item as it is to fit this specific need. Have you considered the Lowepro range, they do bags with a trolley handle loop made in, in various sizes, of which one may meet the size requirements for under seat stowage. I have the Nova 200 AW, reasonabe bag, reasonably rugged, fair amount of padding, reasonably priced, wouldn't go under a seat though so you would need to investigate their range for your self.