I found a Timbuk2 Metro model mini messenger bag in a thrift store and saw the potential to use it as a small pannier bag. I added a couple plastic carabiners to the existing shoulder strap D-rings and it fit perfectly on my Topeak Explorer rack without any further adaptation— truly a 60 second project. I would consider this a fair weather rig as the bag is not waterproof and the top of the bag has a simple flap closure. It makes a perfect size for tools, snack, wind shell and camera.
The carabiners were handy, but it could be attached with webbing straps, or plastic hooks and loops of shock cord.