Greg
Another option that could be less expensive would be the bladders from casks of wine. They weigh 50 grams each and displace 4 litres or 4kg of water and they are quite durable.
The hard part is finding cask wine that is to your liking.

Regards,
Phill

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!