How about the Rubber Roll-Up type like this http://www.gadgets.co.uk/item/USBRK/...-Keyboard.html

OR

This one
http://www.amazon.co.uk/TTJ-Sized-Fl...0354377&sr=8-5

you just need a flat surface like the fold down tray on the back of the seat, although the second keyboard may prove to be a little wide!