How do you map "£" to [RightAlt]+[3]?

view full story

http://askubuntu.com – I've recently acquired a new 84-key US layout keyboard, which has fewer keys than a standard UK layout keyboard. I'm using it in EN-US layout to avoid being unable to type # but I'd still like to be able to type £ without using alt codes and would like to map it to [RightAlt]+[3]. I've managed to map € to [RightAlt]+[4] using Ubuntu's built-in keyboard config tool but I'm a bit stuck on how to go about achieving what I want in Ubuntu 11.10. (HowTos)