I broke off a key in my top box and needed to get a new cylinder and keys. Easy to install and you get two keys with the new cylinder. Depending on which top box you have, Monokey or Monolock, the cylinders might be different. I had the less expensive version and the replacement cylinder with two keys was less than $20, part #076-Z140.
Check which box you have at
http://www.givi.ca/en/replace.php and then order the replacement cylinder and keys through your local dealer. You can also just order the key blanks if you only want those.