Don’t know if this will work for you, but I’ve been making “sandbags” by using a “Noodle” from Wal-Mart (its the long colored foam thingy that kids play with at the swimming pool).
I cut it into sugar cube sized cubes and stuff it into a cut off pants leg tied with string. Granted, this is probably too large for your purpose.
It’s very lightweight and - since a Noodle is designed to float in water - it is totally water repellant. However, while not especially springy, it may not be quite as solid as you need.
Just an idea.