Pat & Garth,
I installed an additional battery on the tongue of"Nuestra Casita" using a plastic marine battery box. It is designed to have a strap around it which keeps the box top in place. To preserve the original leak-proof integrity of the box, & prevent the possibility of bolts corroding underneath the battery, a 1 1/2", "footman loop" was bolted on each side to the box supports. They are outside of, & centered on each long side of the box. The strap was then run through each loop, the box set on the strap and between the loops. The battery was then placed in the box, electrical
cables attached to the battery, lid installed, and strap tightened around the box.
Instead of using the original strap, I used a shortened, ratcheting tie-down strap because that seemed more secure.
Hopefully you can understand this rather lengthy explanation.
If the term "footman loop" is new to you, google it. Most off-road /4wd shops and really well-equipped hardware businesses have them in stock or can order them.
Maybe your imagination can improve on our solution, but this has worked for us in excess of 10,000 miles on the road.
& Ann K.