A fairly common solution is military ammo boxes. They are designed to contain ammo cooking off, as well as protecting their content from fire. They therefore deal with battery fires fine. Just make sure there isn’t anything too flammable close to them, or above them. Any fire that happens will be directed upwards. Just line them with something not electrically conductive.
FYI, never put water on a battery fire. The water can react and make the fire worse. Instead use sand. It reduces flames and acts as a heat dump for the generated heat.
A good setup would be an ammo can for each large battery on a none flammable shelf (metal racking?), with nothing above it that can burn. A fire bucket of sand to hand, to control anything, and a smoke alarm will alert you to any issues as well. It’s overkill, most of the time, but good for peace of mind.
That’s an interesting perspective: I had assumed the biggest risk was while charging, and wanted to set up something to control that risk. I do see one reply about automation to prevent overcharging and I’ll definitely do that, however had not really considered whole in storage.
I’ll have to look into that and how expensive it is for larger batteries