I bought the Knog Franks Dog bag. It has a removable rack attachment so can be used as both a pannier or messenger bag. I looked at the others mentioned here and just couldn't find one that was quite right. This one seemed like it had most of the features I needed. One thing is that I use it more as a messenger bag than as a pannier and it doesn't feel like a kluge. Knog Frank Dog

I didn't pay the retail price on the web site, check Amazon and eBay.