So I built a battery box which fits inbetween the frame rails and on the battery tray. 1 3/8" sink strainers for vents out the back. Wire grommets out the side for power, ground, and tongue jack. ...
Another nuance about battery boxes: If you have one that is sealed on the bottom and the electrolyte spills into the bottom, having nowhere to go, what happens when the sealed box gets filled with rain water? In that case, you would then have a larger volume of water that is acidic.