Thanks for this.
I wood like to think that an ordinary paint
would surfice on the inside as long as all the joints are sealed with expoxy (even Sulfuric acid would take some time to eat its way through 3/4 marine
ply). I've posted the B48 builders inquiring what they use .... everything
is done "top of stds" on a B48.
Also keep in mind the batteries are sealed so the probability of acid leaking from AGM/Gels is significantly reduced (unless the casing cracks which is highly unlikely).
That said, acid-proof paint
on the adjoining ends of the box and the removal
side won't be adequate as I want a "sealing effect" of some sort.