I ordered some metal bearing balls to test as well, though that would require a special metal bowl to allow them to roll in place, or maybe something to coat the ball and metal like surfboard wax (we used that on our sliding metal door when I was a kid).
First I used just a regular balloon, filled it with sand and then blew some air into it. It was expanding too much so I doubled up on the balloons.
Filling the double balloon with 1/3 and 2/3 air seems to be a good combination so far. The latex can still expand slightly and the sand has enough room to roll over itself and stay on the bottom.
I honestly won't be able to properly test this until I have a bigger sphere to work with. Unfortunately that will require a lot more bearings, which are fairly expensive in mass.