I picked up 2 airtight 3.5ish gallon plastic buckets and lids from my supermarket's bakery department for free. I asked on 3 occasions- one time they had a bucket with no lid, but the other two times they had the large buckets and lids I was looking for. One was for doughnut glaze and the other for creme filling. They're a bit of a hassle to wash, but, hey, want do you want for nothing.
I haven't measured it out, but I'm practically positive that a 50 lb. bag of flour will fit in 2 of these large buckets, probably with a little room to spare.
Presently I'm debating whether or not to ask for 2 more buckets so I can store another bag. I go through flour pretty quickly, but, not that quickly. I found an unbromated unbleached bread flour (12.1% protein) from BSM (wingold special) that I want to play around with, but I want to have my old reliable All Trumps on hand as well. Decisions decisions