In my article about Bounty Hunter's Clear Skull Kun, I mentioned filling the figure with stuff, so I filled it with these small glow-in-the-dark tracer BBs from Dick's Sporting Goods. They are plastic and used for air rifles. You can get a canister of them for about 6 bucks.
I bought two canisters, although you may be able to get by with one. I just stuffed as many as possible in there while still being able to get the figure back together.
The figure itself is soft, so you do not really need to heat it up to get the waist separated and back together. It just takes some tucking in when you do put it back.
I filled the legs and the torso/head as well as I could (with a bit more piled onto the torso because the toso's flashing has to fit inside the legs). I brought the two filled halves together, and slowly tilted them toward one another. I had to let a few fall out to allow room for the pieces to fit back together, but I think it filled nicely.
Here's the effect: