Hardware store rubber sheet material is cheap and easy to work with. Depending on your local market, you might have a few choices in thickness. Too thin and the flap will flutter in the wind providing little protection. A step up would be thin conveyor belt. Conveyor belt has a fabric layer which provides structure while retaining flexibility. If you wanna go fancy and don’t mind ongoing maintenance, leather is a great choice. Find your local Tandy Leather (or similar) and dig through the scrap bin.