Seat Pouch or TrunkSack Small rain covercvrP
This one is made for our small saddle bags like the Seat Pouch, old Banana Bag but it'll probably work on lots of other brands' little seat bags.
Also works really well coverin' up TrunkSack Small (aka lil' loafer) bags. But please tape up the seat pouch strap cover "holes" so that water doesn't trickle in that way.
Made out of nylon coated with some type of water proofing stuff.