How can I make a ball for rhythmic gymnastics?

I’m self-taught in rhythmic gymnastics, so I want to have a gymnastic ball, but this is too expensive for me. So I want to buy a cheap rubber ball for kids, and pour over inside with silicon (or what will be better) for weight and elasticity. Can you give me advice on how to do this, and which material I should pour over the ball?

I’m not looking for answers like "it’s a bad idea, just buy a cheap used ball" or "it won’t work". To buy silicon gel really is cheaper in my country. I’m just self-taught, I don’t want to become a professional gymnast, it’s just my hobby.