I have a little aluminum box, which I can't find right now, and can't find for sale anywhere. This thing holds about 4 normal soda crackers. I carried it when caving, many decades ago. It has a thick soft seal, and roller snaps on the ends. It works very well for crackers and I am sure it would be great for flakes. If I can find a small version for sale, I will post it. The large ones are everywhere and look just like the little one I have.
I just stumbled onto these. Although plastic they are well priced and look like they would work.
https://www.amazon.com/Plano-Small-Waterproof-Boxes-Quantity/dp/B000WTYYFG/ref=sr_1_8?s=sporting-goods&ie=UTF8&qid=1512406513&sr=1-8&keywords=Weatherproof+aluminum+box
I just stumbled onto these. Although plastic they are well priced and look like they would work.
https://www.amazon.com/Plano-Small-Waterproof-Boxes-Quantity/dp/B000WTYYFG/ref=sr_1_8?s=sporting-goods&ie=UTF8&qid=1512406513&sr=1-8&keywords=Weatherproof+aluminum+box