Two identical quality made Lipo bags one in the other will provide more then enough protection if a Lipo goes off and possibly starts taking others cells with it......Many quality bags have a temperature ratings many starting at 1000 degrees F on up.......They're designed and would definitely keep fire from emerging from the Safety bags and the bags would naturally vent.....none close that tight........Wouldn't hurt keeping a mid-sized fire extinguisher close by.

Ammo cans and money lockers sound good and fireproof, BUT if the tops are snapped closed & not vented sufficiently they can also turn into shrapnel bombs and FAST!!!......A loose lid will fly open from internal gases letting high temp flames out.

An extinguisher & a single (or more) purchased smoke detector(s) you can hand lay or mount near your charged batteries or charging areas and/or near battery containers at ALL TIMES, might be your greatest comfort when in another room or asleep.....There's a hundred ways you can get peace of mind if you worry about your Lipos possibly going off......and cheap.

And ALWAYS look at your batteries after each use.....ANY dents to the casing is a reason to immediately taking it out of service no matter how long you've had it...... Thanks for reading...JMO