Option1: Some kind of customized dial or thumbwheel number selector, thats not connected to anything... e.g. those combination locks on some briefcases.
Option2: Etch-a-sketch (better than a post-it note, coz it's re-useable). How you can actually fit it to an airplane is anyone's guess, but it's a manual version of option 3.
Option3: A scratch pad, drawing app on your iPhone where you can just scribble in numbers.
Option4: MagicMarker & mini white board.
