It wasn't really raining hard, and I needed my bike for an early morning meeting, so I decided to ride despite the drizzle. I don't mind carrying my leather bag in a light rain, but I was worried it would get a lot of splash up from the road. Also not the end of the world, but I happened to have a plastic bag from a batch of paper towels that we'd just unwrapped at the office. I was taking it home for recycling, and then I looked at it and thought- that's just about the same size as my bag....
So it's not pretty, but it kept the bag completely dry!
I hooked the top of the bag into the clips on my pannier hardware, and locked them down.
A slit in the back is how I slid the whole thing on. Again, not terribly chic, but effective!
On an unrelated note, you know you live in a fairly well developed bike culture when on a rainy night with temps in the 40's, all the bike parking is full. Had to stop at a pharmacy in Harvard Sq, and there was absolutely nowhere to park.