EDIT: On more careful thought, you can just calibrate the scale with the funnel and stand for the ball, then begin filling the ball directly on the scale until it hits your desired weight.
This is a tutorial on how to make Russian juggling balls. I have been using these for years and prefer them over anything. The only downside is that if you plan to make some yourself, you will either have to steal shells from McDonalds or purchase supplies in packs of 100 or 250. Many jugglers use Russians, including Andrew Ruiz and David Ferman.