Mounting battery terminal contacts in an aluminium case
Coat the insides with scratch resistant isolation or fix the contacts such that they cannot touch the metal. Best option. 

Anodisation in aluminium needs to be sealed to be reliable enough for real electric isolation. The anodisation layer can be punctured by sharp edges or worse: The chemicals inside a battery! Some electrolytes have a high or low enough pH or contain sufficient reactive Ions to take away the oxide layer, forming in some cases actually conducting wet-mass for a while. Since I assume you have no crystal ball to see if the batteries will ever leak, it's a decent risk. Anodisation may help retain the coating (due to surface roughness and/or porousness) and/or reduce the layer thickness requirement, depending on the chemical process used.

Many cheap-ish polymer coatings can handle these chemicals and light mechanical pressure, epoxy coatings would be good to a degree I'd expect. (&lt;-- I'd expect... certainties come from spec- and/or datasheets.)

Of course in caps and such the anodisation works, because it's a thoroughly conditioned environment and the anodisation is done under very strictly controlled circumstances until all has been sealed in oil or plastic to prevent thermal cracking or mechanical damage. This is also one of many reasons a cap should not dry out. Dry cap != protected any more.