Originally Posted by
Bekologist
Aardvark waterproof small seems to fit the best.
the selle italia waterproof ones from a couple of years ago are also tops on the list if you can find any NOS.
I've used two types of covers before switching to plastic bags.
The Aardvark cover does indeed fit very well. My only gripe with it is that it's too thick, and the extra padding made the saddle slightly less comfortable during use. Nevertheless, it's not as a big a deal as I've made it sound, and this is the cover I would recommend if the OP insists on using a cover over a plastic bag or shower cap.
The Velox cover is too thin and will abrade at the corners of the saddle until holes appear. Stay away from this one.
I have not tried the Brooks saddle and cannot testify to its effectiveness.