Tailfin. It's an expensive product, but it's gotten very favorable reviews from audax and other ultra riders. They have a few variations (a rack-top bag, panniers, etc) It's designed to work on racing bikes, so it snaps on/off quickly and doesn't require messing with P-clamps.
I was commuting for a while on a road bike using a rack and a pannier. The bike did have rack eyelets, but its geometry was not oriented toward load-carrying. My rack was cantilevered pretty far out back to avoid heel strike, and even a modest load really affected handling. Just something to keep in mind. I'm using a backpack now (which I know doesn't help the OP) and leaving as much stuff at the office as possible. If I had to carry more, I'd look hard at the Tailfin setup.