Theoretically you can fill it with any material. The thing is lead is very dense/heavy so you will need less volume to achieve your target weight for the neck weight. Imagine filling 2kg of sand - that would be a monster thick neck weight. Not very practicable solution.
@@MeugapCommunity Yes totally correct, if the density if lower than water, it will float, even it it weights 2kg or more. The important factor is to use a high density material, to make the weight help you sink more (target of this neck weight)
Depends what you use it for. If you are a freediver probably try to put your total need weight just into this single neck weight to help you maintain a nice streamlined body posture during your descent. The more weight you will distribute towards the head the easier will be the glide downwards - as the body will follow the heavier head. If you are a spearfisher than it will help you to distribute the total weight used (usually more than when freediving) towards the head. Nice to have also less weight around your lower back :)