Good ideas for reusing containers. If you don't mind buying a container, I've had luck with snapware and lock & lock boxes. They are sold as food storage containers, but geocachers swear by them almost as much as ammo boxes. If it can keep a logbook and other contents dry for years on end in the woods, then it will definitely keep food dry.
Both brands have rubber gaskets built into the lids and wings on the sides that lock the lid into place and keep pressure on the gasket to make it air- and watertight.
In my experience, the lock & lock are built better and lock into place better. The snapware are decent, though, and a little cheaper.
I have found both at Target and Meijer, with Meijer being cheaper. If you look online, Lock & Lock have lots of neat different sizes and shapes.
Normal disclaimer applies - I have no affiliation with any plastic container company, I just hide them in the woods and stick food in them on occasion.