Have a look at the Kriega stuff. They do a basic smallish tailpack of about 30ltr but you can buy two addtional smaller packs which attached to the 30ltr. From memory I think the attachables were 10ltr each.
http://www.kriega.com/pages/us20/US20-topframeset.html
www.kriega.com/pages/us10/us10-frameset.html
I saw it at the Excell show and it looks really good.