Torklift makes a great battery box. I have their double battery box with sliding lid (mine is similar to the Power Armor DH 26). Since they are all aluminum, weight is low and they have one of the best thought out locking systems.
Presently, I have a repurposed steel tool box on the tongue and a dual 6V DIY box inside the trailer. I am trying to keep my batteries close to the distribution center and off the tongue for weight considerations.
BTW, install a disconnect switch on your new box if you don't already have one. It will save you a lot of battery longevity.