I plan on buying a 55lb bag of grain soon and I am going to do exactly what the preppers do for this kind of medium to long term storage. It' a grain, so just get a mylar bag and some oxygen absorbers; place grain in mylar bag, put in O2 absorber, seal with an iron. Done. Should be good for years stored in a cool, dark, dry place. Not saying it is going to last for years... but still - good piece of mind.
Worth mentioning too; instead of doing a big 5 gallon bucket worth of grain, I plan on breaking up the 55lb bag into 5-10lb allotments. I typically use 10lbs of base grain when I brew so just having them portioned out like that, I can just grab a bag and know that :
1) all my other grain that I am not using is still sealed for the long haul [not having to crack the seal on more grain than needed]
2) I already have a 10lb measured base grain. Just need to grind that up.
Also, when you use the mylar, you can store them in a orange HD bucket with no worries of getting chemicals leaching into the grain (at least that is my understanding after doing some research).
All this said, I have not yet done this but bought my mylar last night so maybe a couple weeks out from doing this.