what about water?
What about shipping empty hard shell containers that interlock into a cylinder and filling the containers with water. As more storage is required more devices can be added.
In a cylinder of water with dimensions 2m radius, 10m high, rotating at 5,000 rpm the kinectic energy is around 34 mega watts.
The only problem I see is that the centrifical force on the outside of the container would be close to 7,000 tons and it would have a static weight of 125 tons All calculations do not take into account the container.
As for balancing, if water is used a automatic system to pump water from one chamber to the other to achieve an intial balanced state could be created.
The only problem I see is that it is too simple for the DARPA bofins.