2x2 piece of wood, 18-24" long.
Cut in 1/2 about 50-70% up the piece of wood.
Put a hinge at the cut point and a small nail at the top. 2 small nails to wind line on to.
Put the line down the hole and make a small loop knot with a washer on it. put the washer on the nail at the top.
When the line is pulled by a fish, the top half will flip over making a slapping sound to alert you to a fish on the end of your line.