I live in Washington DC. one of those Humidity prone area of the country.
I've always stored my BP motors in boxes i've made out of other things (a Neon Transformer box, an Ammo Can and a plastic Long term Storage foot locker that contain my unopened stock.
I regularly fly OOP motors from a far back as the mid 1960's. and have a rather extensive stock of these Oop motors that have been in my possesion all these years.
In all the years I've flown in the DEL-MAR-VA area, I've never had a bit of problem with motor humidity swelling or any other weather related problem other then HEAT CYCLEING which is the only real BP motor problem.
As a matter of fact I've conducted my own tests by purposely Heat Cycled a number of different MMX, 13, 18 &24mm BP motors, leaving them in my Van for a couple years to cycle from 0-degrees in the winter to +155degs in mid summer. Of those motors I've only been able to Cato a few of them by flying them outside the 75deg mark of the hottest temperature to which they have been exposed. ie 155degs - 75 = 80def F. As long as I fly these marked motors at 80 degrees or higher they have preformed as designed. I have Cato'ed a couple D12-5's flying them in the 75degree range. Funny thing is I have been unable to Cato a MMX 7.1mm motor even after Heat cycleing them for 4 years straight in my van and flying them in winter in the mid 30 deg temps.
Conclusion: As long as we don't HEAT CYCLE our BP motors they will last indefinitely. If your motors are kept in a climate controlled house or garage you shouldn't have to worry about them at all.
Heat Cycling is really the only problem area for BP motors. I hear folks talking about motors swelling with humdity and have to say BS! the only humidity swelling would take 100% humidity for weeks. Sorry folks; I've tried very hard to make "Humidity" a factor in BP motor longevity but have been unable to prove or observe any "Moisture" related problem, at least in my humid neck of the woods.
If any are looking for inexpensive 30cal ammo cans: Harbor Freight has some all Plastic rubbler hinged lip sealing cans for 4.99 each at the moment. I use them for long term ammo storage...not motors but if you really think you need ammo boxes these do the job at a really reasonable price