Early LI batteries had a tendency to FLAME ON due to crappy control systems.
Newer ones .. Well any battery can do it Even lead acid. but the odds have gone way way down.... I used to sleep atop a Lead Acid and still sleep beside 3 of them.. When they get replaced I'll be going LiFePo4
I'd go with the current battery location.. If all else fails a good metal smith can easily weld up a battery box for you.. just 5 flat plates some holes and a lid. the lid can be other than metal if you like. Well painted plywood and 1x2 edges.