Here's a place that makes add-on tanks. Tour Buddy They aren't as purdy as the spun aluminum jobs, but they are very useful and it's a mom & pop kinda operation. These people will bend over backwards to help you out. I've used different ones on snowmobiles and the Tour Buddy's work flawless.
An old mate of mine adapted an old Virago 535 tank to fit into his frame to give him more range. He took out the air box and really mucked about with it even had to cut out and reweld part of the frame. Don't now what he did with it, I'll ask him if he has any pictures.