I bought a Mountain Hardware Enterprise for a little larger bag, and it does the job.
http://www.amazon.com/Enterprise-Pac.../dp/B000FDULP6
I can fit a change of clothes and my lunch for my ride to work, plus some.
It has a pouch area for a laptop too (hence the term "Hardware"), but I haven't found the need to use that area for anything other than notebooks.
It also has two side pouches for water bottles, which I thought was important.