Just my two cents
My wife and daughter swear by the Away bags, they have two in carry on size and perhaps the smaller of the two would work for your needs. I would mention that increasingly I see the airlines using the underseat space for electronics boxes and the like and so rarely if ever can I fit more than a small, light backpack. My issue with the Away bags is the same I have for all the Rimowa bags. If you need to open you bag to get something out it becomes a pain since you have to unfold the bag and there is zero space to do that on a plane. My wife cures that problem by putting anything she could want or need in a small backpack and that goes under the seat. The rest goes in the Away bag and she just plops the backpack on the Away bag when moving through the airport...