This is what I've been using (Bontrager Interchange Grocery bag): http://bontrager.com/model/08102

Most of the time I just toss my backpack inside.

I've been keeping my eyes open for something I'll like more, but I've yet to find something I really like. Well, I was pretty interested in the Linus Market Bag, but I did some reading and apparently some people have worn through the canvas pretty fast.

At this point, I'm thinking of just adding mounting hardware to the non-strap side of a backpack.