Wow! I got depressed when I was speccing out a UPS a few years ago because they all suck (use Lead Acid batteries) but there are many practical reasons that they don't use better tech.
You raise a good point! I spent some time thinking about this. Ultimately, it wasn't clear to me that any plastic I could reasonably work with would be more fire-retarding than the dense wood that I ended up using, and of course a metal enclosure would be a far greater fire hazard due to the increased risk of short circuits.
I would disagree regarding the metal box. Sure, it can cause short circuits, but that’s relatively easy to guard against with proper insulation. Even if shorts do occur, it will blow a fuse or trip the overcurrent protection. Even if it doesn’t, it won’t provide a supply of combustible material to sustain a fire for long enough to ignite the nearby batteries.