Waterproof seat cover - source?

Hi all!

It's starting to get rainy/dewy here in the bay area. I am looking for a waterproof seat cover to keep my butt from getting wet from the seat. I once got a freebie type w/ elastic borders that fit PERFECTLY around the BayWheels bike seats, but I have since lost it and have been using heavy duty plastic bags. Those work fine but less cute.

Does anyone have a good source? Alternately, anyone up for making one and putting it on etsy?? I'd be a customer!