A chunk of dry ice would about do the job.
Cheap, readily available, and perfectly legal. The better pelican style cases have a purge valve to deal with the fact the excellent seals can provide a healthy positive or negative pressure. Handy if you are trying to unpack your NVG's you packed at sea level before a jump and don't want to get yelled at. Also fun to get the case back open when you land, if hard on people nerves.
Dry ice would up that considerably, and if you just popped the latches, could result in excitement and minor injuries when the seals blew out. The case for a VR headset could easily be 2-4 L, so that's a fair amount of gas, not quite exploding soda bottle levels probably, but in the same ball park.
Still, probably not as bad as if your fellow PFCs tossed in a couple of MRE heaters and some Tabasco sauce.