Hi,
I recently bought a bag from Bags-connection.com. very nice bag. It cost me 289 Euro, and fit nicely on the back of my bike. Very durable. The only downfall was, I had to take it off everytime I wanted to fuel, which it only took a minute. I have a small backrest on my bike, which supported it from sliding backwards, though the straps around the shocks would have held it, and it provided a nice back rest too.
Hillbill