Topeak makes a yellow rain cover for $6 at REI, which rolls up and takes very little space. Doesn't fit the pannier version, but there may be a cover for that, as well. Definitely works best with fenders, as the bag is not waterproof

Use a bungee around the back to keep the bag secure from an accidental release and from rattling, which doubles to secure a stuff sack on top, when needed. You do need to be careful to make sure the bag is in the track.