A simple no nonsense screw top tube with a liquid proof seal. Approx 52mm long they can be hidden almost anywhere and if the need arose you could 'deep carry' it to retreive it later. Alternatively, could also be 'stashed' externally easily, in a variety of other locations.
Pearl walls allow you to improvise a light with close glow.
A perfect addition for storing water sterilisation tablets in a survival kit for example, possibly one of the few things that has a hundred uses and the more you use it, the more you will find.