I wonder if you could get the command adhesive strips/hooks for hanging pictures and affix them to the Otterbox case, attach a wire as if it was a picture and hang it using the knob that holds the tray in place? They have a wide variety of different hooks, attachments, so I believe there are some that are small enough where they wouldn't be unrealistic for the case. I don't like any of the ideas that will impact the passenger sitting in front of you, and feel that the string would do just that. Plus, if they decide to tilt their seat, that is going to affect the viewing experience.

Just an idea, I haven't tried it, because I usually prop it up on the tray table. I hope you find a solution, I can see where it would be useful, but don't see the point of buying anything expensive like a special case or holder for that purpose.