I had one of those wallbike velox covers for a b17, it served primarily to wick water off the waterproof top around the edges straight into the the sides of the saddle. Do not recommend. One of the Brooks ones (middle of 3 images on that link) came w/ my latest saddle and it seems to work great. But I wouldn't buy one after the fact.

Otherwise definitely a plastic shopping bag, w/ a rubber band to keep it on the bike if I park it in the wind.