Actually we don't have an explosion in such cases, unless we create one. The failed battery heats rapidly, boils it's contents making lots of the wrong kind of vapor which raises the pressure enough to open the SAFETY VENT preventing an explosion. Unless we again trap the gas, like in an 'airtight' case........
Explosions happen in two cases. The first is where the 'burn rate' (measured in feet per second) is faster than the speed of sound. This (being faster than SOS) means the expanding gas 'can't get out of the way fasts enough' so more pressure builds behind it. Things like TNT burn 10 or more thousand feet per second against about 1100 fps for the SOS. These are 'high explosives' a pile of them on the ground is dangerous, extremely so usually.
Things line gunpowder and our little 18650 'steam generator'
only explode if confined in a jacket so the pressure can build up. Empty the contents of a pipe bomb into an open top tin can and toss in a match and you have a festive little fire for several seconds that does no damage at all since the gas is liberated over time and not compressed. In the pipe bomb case, or confined within a firearm, the increase in pressure makes it burn faster in fact, small fractions of a second. This (lack of confinement) is the reason firearms ammunition goes pop when thrown into a campfire, not boom (don't do this at home.......or in the woods......or at all).
So putting them in a gas tight case that can shatter is a bad thing. Turns a non accident (venting) into a potential explosion involving shards of hard plastic. Truly nasty wounds, since they don't show up with X-rays in the ER nor with the metal detectors they look for more conventional shrapnel with.
I'd suggest shifting to cases designed for the task, ones designed to safely vent in the case of mishap, for instance:
http://www.amazon.com/gp/product/B008HL8CW2?psc=1&redirect=true&ref_=oh_aui_search_detailpage
I store extra herb, ground and ready to go, on the other side of the case usually. I use a sawed off 'doob tube' which you can get from PIU or PV for a dollar or so. Handy little (gas tight) containers. If you want water tight but still safe that's available too:
http://www.amazon.com/Case-Star-Pla...=8-14&keywords=Battery+Storage+Case+for+18650
If you're going to use hard plastic sealed tubes like the match safes I'd drill a vent hole in it. Or maybe two.
OF