On my old trailer I added a new battery box where the original single box was located. To fit the width of the space the new batteries are side by side.
I clamped square tubing to the angle iron brackets and drilled holes through the tubing and the brackets. I held the new box in place and drilled up through the brackets and the bottom of the box for the bolts. Putting the bolts in as I went to keep the box from moving. The bolts are in the low spots of the box and don't touch the batteries. I had to go back later and put drain holes in the bottom to keep water from building up in the box from traveling in the rain.
The last picture is after I added the disconnect switch the next year.
This is the battery box I used:
https://allbatterysalesandservice.co.../category/760/
Gary