Is that really an unsolved problem in airline business? It can't be that hard to build small containers that fit on the seat and can be fastened with the seat b

Is that really an unsolved problem in airline business? It can't be that hard to build small containers that fit on the seat and can be fastened with the seat belt system. Obviously, you won't want to do this for general cargo transport, but for "hey, we have a grounded plane, but transporting air mail gives great rates right now" situations?