Thank you very much for sharing your instructions.

I modified the method a little bit. I do not have to cut an extra hole into the balls, because I use the small hole to fill in quartz sand with a self-made funnel.

You can use any disposable plastic cup or paper coffee mug. Drill a small hole into the bottom of it. Then place the bottom of the mug with the hole on top of ball and make making sure that the hole in the bottom of the mug is exactly covering the small existing hole of the ball. Now just fill in the desired amount of quartz sand.

Afterwards I sealed the small hole with a tiny piece of plastic from a ball I cut in half using a temperature controlled soldering iron (adjusted to 150°C).