I went from single 12 volt to 2 12 volts. Took out the original box. Measured and got a regular plastic storage box. Cut off the vent from the original single box, and added it to the storage box top. Added some foam tape I had laying around, to the top lip of the storage box, to provide a better seal. Cut holes in the bottom to the outside for air flow (same spot as the original single box).
Secured, put both batteries in, and hooked up.
Two boxes would have taken too much room. Double box was too big and expensive.