Alot of people have used thewindow bungs from citroen AX's (myself included. best bet is to go to a scrap yard and have a look for any type of bung for this. Some old lower trim level fiestas have a bung that should fit as they wernt fitted with a rear wiper.
Other wise measure the hole (think it'll be 25-30mm) and google '25mm blanking grommet' or whatever size it is.
what you could use is a closed grommet.these are used by sparkys to close holes in electrical cabinets.i know you can get a 25mm one. try your local electrical supplier.you can get open ones, that are used to protect cable sheathing when a hole is cut in a metal cabinet.