I installed the Ventura luggage kit on my Blast -- a 51 liter bag, mounting hardware, everything you need. Costs about $350, but, to me, worth every penny.
The bag is more than big enough, but they also sell smaller bags. If they aren't big enough, they have it so you can zip the backs of two bags together.
The system mounts on a "sissy bar" style mount - the bag slips over the mount and ties down on the carrier -- ultra stable.
When not in use, the bag that comes with the kit includes "backpack straps". Additional bags you order may have to buy the straps separately.
Installation is a breeze. A good set of instructions ships with it. All you need is a wrench and a drill (the drill is used only to enlarge two holes in the plastic license plate bracket -- very easy). Should only take a half hour only if you are as mechanically incompetant as I. Otherwise, 15 minutes top.