See the link that Ells posted above (as you're not the first to ask!) BUT just about anything can be made to fit with a little bit of ingenuity (and cable ties and duct tape!).
I've previously had a Triumph race seat unit on my 600/620 SS as it was less than £30 off ebay. Seat units from an RS250 are also quite a good fit too.
Put it on, mark it in pen, attack it with a saw and try it. Keep going until it fits!
Half the fun is having something different from everyone else!
