Get your friendly garage to steam clean (inside and out) and, to be really flash - repaint, 5 good quality (as new as possible), 44 gal fuel drums. Fit new O rings / seals to the caps and you are in clover.
One of these filled with 200 litres of water is as near as damn it 220kg. 5 fit nicely on a standard pallet in a compact star and can be easily and securely lashed.
Load that in the appropriate hold space to trim. Empty weight for transportation around 100kgs for the 5 drums + pallet and strops.
This will not cost a fortune and correctly marked / painted they can be re-used as water ballest as often as you need. They do rust out eventually - but it takes a few years.
MG