Just use MTM plastic ammo boxes. There's no legal requirement that they be labelled and there's no legal requirement that they be factory loads.

There is a legal requirement that ammo be packed in cases that keep every round seperated from every other round. - So the MTM boxes are fine for that. Some airlines also require that those ammo boxes also be packed into a metal, lockable container...... this isn't a legal requirement, it's an internal regulation, but you need to check that with your airline.

The other legal requirement is that you take no more than 5 Kgs of ammo per passenger.

Hope that helps