For my '72 Sovereign
, I built a battery compartment for two batteries on the bottom of the streetside closet. The batteries are housed in a thick plastic tote and these vent out through a plastic tube. My blog has pics and details of this configuration. Truth be told, I don't like putting these in the back, I much prefer to place my batteries up front, like on the tongue, but mounting them there would interfere with the spare tire mechanism.
For my '58 Overlander, I will be placing the two batteries on the tonque. My latest blog entry has a pic of the A-Frame with the new battery mounting.