It Impulse in Oz, we used to use those 20 litre drims that chemicals come in, usually Cholorine for pools, but filled with water. The handlse make convenient tiedown rings too.
Filled almost to the top with water, each one weighs 20 kg. When you donn't need them for ballast. Simply tip out the water on the ramp and they weigh next to nothing.
Easy....