Find a nice ratty looking piece of tupperware with a tightsealing see-through but semi-opaque lid. Find another one that fits inside the first one with at least an inch of space to spare around the sides, but with as little space at the top as you can get away keeping in mind that there needs to be enough space that both lids will seal firmly, with about 1/8 inch of headroom. Download, size and print this to whatever dimension will fit the lid of the bigger container. Cut the image to size plus a quarter inch all round. Crumple it into a ball, then smooth it just enough so you can glue it to the inside of the lid with rubber cement, and here's the secret; pile the rubber cement onto the underside of the lid super gloppy, so there's random pools of it, then stick your paper down. Turn it so you are looking down at your lid, and behold the beauty of some gross unidentifiable food stuff topped with rubber cement gravy. Mmmmmmmm! Put your smaller container inside, and put something heavy like a rock or soupcan in it to hold it down, centre it, and pour whatever stewy chewy liquid food you have that most resembles the grossness on the lid in the space around it. Fill it up to the height of your inner container. Throw in freezer, and when it's frozen good and solid, voila! You got yourself a water and fire-resistent stash box for your stuff. Mind you, now all the other monkeys can come over and rob you blind. Make them talk to you through the door and serve them cookies through the mail slot.
Ingenious George' s Freezerburnt Secret ...Cool...and easy to make for free.