LPS-3 Rust Inhibitor comes in can or spray contaners.
A couple coats leaves a hard film that will protect machined surfaces.
At jobs I have worked we have used LPS-3 to protect machined surfaces on
equipment that is stored outside under nothing more than a tarp for up to 6 months.
The stuff looked like the day we left it when the tarp came off.
I like Daves' idea of building the ELEVATED storage boxes. Elevated meaning building on top of sturdy pallets
so that when the room gets flooded by an unexpected rain your tools won't be sitting in 3 inches of water.
Plus the pallets will make them easier to move around with a machine such as a Bobcat or front end loader
with forks or a pallet jack.
If there was a way to keep the box slightly warmer that would help to drive moisture away too. BUT...it would have to
be some safe way to heat the box, not a light bulb or some other makeshift form of heater, because you won't be
monitoring it and a failure could cause a fire which would negate the whole purpose wouldn't it. but there are low
wattage pad heaters which might work for the winter months. You'd have to do some research to find something that
would work. I think the moisture absorbant packs would be overcome in only a day or so unless you boxes are fairly air tight.