This "end to end" battery box I purchased from my local "Allied Battery" distributor. This is the same place from which the batteries I bought came from. The inside measurements of the box fit perfectly with the batteries within. Along the bottom of the box I drilled, then mounted it to the support rack using sheet metal screws. The wires leading into the box enter from behind, just beyond view, thru access holes just like the one visible on this visible side.

After I attached the battery box to the rack, I went to "REI" and had them custom make me 2 ea. straps with buckles to wrap around battery box, rack, and box lid.

The image to the left simply shows width of the battery box in relation to available space between trailer frame rails.