This might sound really odd but it might work. If you cut a circle or maybe a square of net out of an old curtain, the 'diameter' needs to be quite a bit bigger than the top of the water butt, so that it overhangs. Then to weight it down, use stones by tying a knot near the edges and placing a stone in it or put the net over and hold it in place with a piece of strong elastic knotted into a circle. Is there a prize for the most crazy idea ?