Clothespins can be a great help, too! They let you easily turn a spare poncho or clear plastic garbage bag into a customized cover. Best of all, they don’t damage a makeshift covering so it can be used multiple times. I always have some in my park bag.
I bring a dozen or so clothespin in a suitcase anywhere I travel. They make ideal clips for food packages (chip & cookie bags and more) and can even hang a piece of laundry of two.