I just use a plain old canvas backpack with the leather bottom available at your local Target/Wal-Mart/etc. I think it's the same one I have had for about 10 years. It works fine and fits comfortably over my jacket. The biggest thing I have put in there was a 12-pack of soda. It was heavy, but not unmanageable
Just a safety note when using a backpack. Always put the zippers down in the corner and never in the center at the top. On mine, the outer compartment zips to the right side, so I put the main zipper down in the right corner. That way, I can check that both zippers are ok with one look in the mirror. Not that it has happened, but sometimes, you get that "did I zip it all the way closed" feeling and this makes it easier to check. So far <fx:knocking on wood>, I have never had a backpack come open on me in the 17 years I have been riding.