The problem with FZS600's is the shape of the back end.Most of the racks available are the sports type
which will only sit on half of the carrier,therefore you would need to make a strong bracket to secure a
topbox.
I tend to use throwovers as they are less hassle,and a sports rack (Yamaha) with a small tail pack.
KAPPA and GIVI make racks and side rails but they are extremely expensive - £280 plus.Check out the bay,or "for sales".You may be lucky!.