in just robbed one of my plastic screw/bolt thingies from the rain tray belive it or not, luckily i found a bit of rubber with a hole in one end that fitted snug in the wiper hole and then i inserted the screw thing in the hole in the rubber, this made it expand and its totally water tight
btw my raintray is absolutely shagged and need a new one so it was ok to butcher it up lol, not so good if yours is good condition like haha