I use a large (half gallon,maybe?) old ziplock bag on any saddle I use in the rain. The nose goes in one corner, bag is pulled back, and all the excess is stuffed up under the rails. The ziplock part doesn't matter-this bag is just heavy and large enough to do the job. It stays put with no tape, too. Usually lives in my Carradice, but goes in a jersey pocket if the weather is iffy.
Best Regards,
Wes Gadd