I use the daypack II on mine from bags connection.
It is expandable, has a waterproof cover, attachable atlas holder and is reinforced to keep it's shape.
It uses a tank ring so no scratching the bike and can also be locked to the bike for security.
Easy removal for refuelling as well.
I am very happy with it, and it's not bulky.
I carry alot in there eg wallet, gps case, tissues, cat crap and cloth, remote for garage door, keys for panniers etc, sunnies case, instant camera and haven't had to expand it yet so it can take quite alot.
Install takes about 10 minutes.
Hope this helps
map holder (removable)
side view of who it looks with the all the bike in view