To mount my batteries, I took a couple 1" aluminum angles, screwed them to the floor with a half dozen #8x5/8" screws with just enough space between them for the two batteries, they each have a horizontal slot cut in the vertical portion, that I threaded a cargo strap through. 2 Batteries snugly fit betwen these two angle irons, strap over the top keeps them quite secure.
(I added a few more screws after this pic was taken, and no, I didn't put the fuse holders there)