As these cursed cannonballs aren't going to be as common as regular cannonballs why not simply use crates to store them?

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Even the sloop has thirteen empty useless barrels which could be replaced with either functional cursed cannonball barrels or space saving crates.
One of each colour type (purple / green) would suffice.

Why the need to over-complicate things is beyond me.

The crates could be non transportable, although stealing the whole crate would add another treasure worth looting.

edit: Here's my suggestion for selecting cannonballs to use