First off let me say that this was not an original idea. I saw it on some outdoorsman's blog and decided to tweak it just a little bit.
2 used plastic shotgun shells
Craft knife or Box Cutter
Candle, Heat Gun or some other safe way to warm the plastic.
Step 1: Choose the Shotgun Shells
Choose 2 shells that are of the same gauge (caliber is not the correct term for shotguns). Inspect to make sure they are intact and have not been re used too many times and don't have any holes, cracks or voids in the plastic or metal parts.
Step 2: Cut off unnecessary crimped end
Choose one of the spent shells and cut off the crimped end with a box cutter or band saw. Hover over the image below to see where I cut. I'll call this the waist as it is the narrowest portion of the shell. It will be readily apparent why in a later step. Do this only to one shell. On the other shell it isn't so critical where to cut.
Step 3: Fit one shell in the other
Using a heat gun or candle, warm up the shell without a waist. As the plastic gets a little softer jam it over the other shell with the waist still attached. The narrower tip of the shell will make it easier to slide into the warmed shell. Shove it in as far as it will go. Set the two shells aside for a couple of minutes to cool.
Step 4: Unkork
Once cool pull the shells apart. It will be difficult, but it will happen eventually. You might need to squeeze the center of the shells to loosen them. There will be a popping sound due to the air tight nature of the new enclosure. The air was also heated inside the shells and as it cooled down caused a bit of a vacuum.
Step 5: Finish the project
Once the shells are apart, cut the narrower one (the one with the waist that slips inside the other) in half. This will make it a bit easier to pull apart in the future. If this container is meant to keep items dry for long periods of time (like matches in a Bug Out Bag) that you won't be accessing frequently you may opt to skip cutting the shell shorter. It will make it harder to open, but will keep moisture out even better.
I mentioned at the beginning that I saw this idea somewhere before. I would like to give credit to the site where I read it, but can not remember what it is. I'll keep looking for it and update this instructible with a link to his site when I find it. The difference between our ideas is that he never heated the outer shell, instead he slitted the side of the inner shell so it would slide in the other shell. I like my version better, but I would like to give credit where credit is due.