I've got one that sits in the top pocket but it's not for waterproofing. Mine is day-glo orange and has adhesive scotchbrite reflective tape on it. It was originally just for use in an emergency if I needed to be found, but I've also used it when walking on unlit country roads at night for extra visibility.
I cant remember where the cover came from, but I think it was just a high street outdoors store. I'll have a think if you're interested...