Lets begin with a clean 2 quart soda bottle. Add small rocks to the bottom for weight and stuff with plastic grocery bags to keep the bottle from caving in.
Next, put a four inch styrofoam ball inside a plastic bag wrapping the bag around the ball, securing tightly in a knot. Insert the ends of the bag down into the opening of the bottle.
Fold a plastic bag lengthwise and wrap around the neck of the bottle. Secure with masking tape.