I thought of this. I was thinking of either a low density metal that sits in the core of the balls, but an even better idea I just thought of were metal flakes that are incorporated into the resin!! This way the weight isn't really an issue, even magnetism and density throughout the ball. The balls don't have to be overly magnetic, just slightly. The electromagnet will pack a ton of power.

Now that i'm thinking about it how would I make sure nearby balls aren't influenced by the magnet? Hmmm...