With my new job my commute is about to increase quite a bit and I was thinking about getting one of these:

http://www.timbuk2.com/tb2/products/...nier-messenger

It might work for your needs too. I have one of the "medium" size messenger bags and you would be surprised by how much you can fit in a 'medium'.

You could always just strap your current backpack onto your rack. I did that for quite some time with just an elastic mesh net I found at the hardware store.