Take a big balloon and fill it with a small amount of water (I'd guess maybe a little less than the volume of a baseball), make sure all the air is squeezed out, and tie it off. A big balloon (12" party type, not a 'water bomb') is used to keep it loose and squishy rather than firm. And the air MUST be completely squeezed out or the sloshing water will make noise. Tucked into a dance belt, the result is a very natural-looking amendment to one's endowment that molds over and moves and jiggles like the real thing rather than looking stuffed. Try progressively increasing the size slightly each time...see how much you can believably get away with!