Hospitals use small plastic sterile evacuated tubes for blood collection. some have chemicals in them-heparin, edta, or a gel to preserve, anti-coagulate, or separate the serum for lab use-but the red ones are usually chemical free, and will hold a single column of motrin. It's a waterproof, crush resistant, relatively compact solution. These items have expiration dates on them, and are often discarded as they out-date-usually can be had for the asking. I have also fabricated some waterproof containers from empty liter IV fluid bags-cut the top off, dry, add FAK, fold the top over three times and clamp with binder clips. it's a low cost version of the aloksak technology, and has proven more durable than similar contrivances using ziplok bags.