custom region and faction restrictions

Hi, my question is about how to set custom province and faction restrictions.

For example, I want to mod the sicilians to be able to get some muslim units like desert archers and nubian spearmen, but only in malta.

My current solution is to extend the units to be available to the sicilians and make malta available for these units.

However, this make the Sicilians able to get those units in the desert regions as well and the muslims can now get those units in malta if they conquer it.

Any ideas?

You might be able to copy the unit and make it so that it's only available to the sicilians and only available in Malta, although I don't know if this would work as I've never tried it...