I would have thought you don't actually need exact shape, just a bit of a bulge in your pants. I think you would get a better result in tight underwear. That way you could stuff anything that came to hand in their to get the right amount of bulge - tissues, another pair of underwear, socks.
I think you need something smaller than fist sized. Guys don't tend to have that much going on when flacid, think something like those large nitted hacky sacks...