These two 16" tall @ 6v each (L-16s) weigh a bit more than our 18 hp engine!
(Over 300#s)! I made a 3/4" ply box that was a bit larger than needed. It was pre glassed, so that epoxy
fillets inside, and glass corner tapes outside were all that was necessary to finish it out.
To fasten down the box, I used 1.5" X 1.5" angle aluminum
strips at the bottom, and to fasten down the batteries themselves, aluminum
flatbar across the top. This flatbar is fastened down with 2" X 1/4" machine screws that are threaded into the 3/4" thick walls of the box. (I had poured a solid epoxy
spot, drilled, and tapped the 1/4" threads, > 2"deep, to accommodate these machine screws.) The flat bar itself was padded with rubber underneath, for a jamb tight fit.
The bottom and sides of the box are lined with 1" X 1/2" thick strips of starboard, about 3" apart, to provide ventilation all around, keep the batteries cool, and create a jamb fit!
We have fallen off of a mountainous wave that broke out from under us, dropping the boat a considerable distance and landing on one ama. The box was fine! With a weaker box... the batteries might have kept going!
You are correct to be concerned about the issue... Best of luck with it.