How about having an ammo box sold at one of the vendors for about 1000g.
The box can hold between 50 and 100 shots, is refilled by holding it and interacting with a standard ammo box.
The portable ammo box can be carried to anywhere and placed anywhere a cage item can be dropped, it can be used by enemy crews as well.
You should be able to buy as many ammo boxes as you want or maybe have a limited stock at vendors per server, which maybe refreshes every hour or so.
If you leave your ammo box on an island it can be taken by another crew, it will eventually despawn maybe set to the same timer as dropped cargo?
What do you think?

