dual battery install, and a roof coating ?

You'll need to support a new battery holder with steel supports underneath. There is just too much weight for any type of attachment to a vertical wood wall. I'd suggest that you remove the bottom plastic piece of the enclosure, weld (or bolt) steel cross members onto the tongue and then replace the plastic shelf. That is the easiest way.
If it was me, I'd install a locking battery box and move the tanks forward on their own purpose built tank holder. In fact, that's just what I did.

Roof coating first, then module installation.