[PapaBear3625]

One winter (since the air was low humidity anyway) I put the ammo I intended to store long-term in plastic zip-lock backs and put them in sealable zip-lock bag, and the bags in .50-cal ammo cans. I figure that, with a good seal, more humidity can’t get in.

[Stormdog]

For short money, get a food vacuum sealer. It works well with food too.

[45semi]

I drink a lot of Country-Time lemonade (prevents kidney stones dontchaknow!)
and it comes as a powder in a round, hard plastic container that fills exactly to the brim when a box of 500 .22lr rounds is poured in.
It has a screw-top lid, and twists shut so tight you know no moisture will get in.
Gatorade powder also comes in the same type container.
If you were really worried, I guess a bead of silicone sealant around the threads first would make it entirely waterproof.
Maybe some rice in a strip of panthose would take care of any moisture sealed inside.