you can also go for the kriega + tank adaptor
http://www.kriega.com/pages_uk/us5/US5-topframeset.html
http://www.kriega.com/pages_uk/us10/us10-frameset.html
http://www.kriega.com/pages_uk/us20/...pframeset.html
http://www.kriega.com/pages_uk/tank/tank-frameset.html
i use the US5 on a daily basis to store lock and stuff, and the US10 for longer trip.
I didn't buy the US20 as I judged it was too big to use as a tank bag
I'm very happy with it.