Seal in .50 cal. GI ammo cans. Keep in a cool place where the temps don’t vary much.
Loose boxes can be just kept in your house but not in basement.
Probably not necessary but desiccant in the cans would be nice.
I have found the Plano brand of opaque plastic storage boxes with a rubber gasket work well. Desciccant packages certainly won’t hurt.
“Seal in .50 cal. GI ammo cans...”
50 cal cans aren’t any better than other sizes, but their larger internal dimensions do afford more flexibility.
The older style cans fabricated of drawn steel are stronger than the newer plastic variety, but are at least as heavy. Corrosion risks to the steel cans are not zero: both inside (if dessicant is neglected, or they get packed on a very humid day) and outside - if the cans get shelved where moisture (or worse, pooled water) touches the bottoms.
The strength of steel can come in handy, if one must stack supplies.
Black powder boasts a long shelf life, if kept dry: it’s merely a mechanical mixture of the ingredients (potassium nitrate, sulfur, and charcoal) which can almost never separate, thanks to modern mixing, drying, and coating methods. Nitro propellant will ultimately degrade: the solvent chemicals and mixing/reactant agents can never be completely removed following formulation Cool-and-dry is recommended, because it slows any incipient reactions as much as they can be.
For decades, the US War Dept refused to use non-corrosive primers, citing insufficient evidence they’d remain stable. The sole exception during the Second World War was 30 US Carbine ammunition, which was specified from the outset to use non-corrosive priming. Shelf life can extend well beyond “a few decades”: in a prior job, I repaired guns and test-fired M1 Carbines. Less than five years ago, I touched off a couple 30 Carbine rounds headstamped “44”: zero hangfires, zero misfires.