Swim cap bicycle seat cover

Do you have an old gel-padded bicycle seat with fabric peeling away from it, that turns into a sponge when it rains?  Perhaps you have a seat similar to this on your commuter bike.  Are you tiring of using plastic bags to cover the seat after it rains to keep your pants from getting soaking on your ride?

If you happen to have a collection of swim caps from old triathlons you may have raced years ago you might try that as a bicycle seat cover.  They are certainly stretchy enough and will certainly keep your butt from sliding on the seat (although it may be more grip than you want).  Give it a shot.