I ended up buying Rubbermaid bins with lids. They're roughly squareish with rounded corners and sort of wider at the top than the bottom. When I bought them I was trying to guesstimate how many "gallons" you'd get from a pound of barley malt. Reason being, those Rubbermaid and other storage containers like trash cans etc, are all labeled with how many gallons they hold (in the US, elsewhere I presume liters) 13, 33, 45, 60 etc.
I was at work at the time when I wanted to hit the hardware store so I went searching online everywhere for a decent conversion to tell me the volume of a pound of whole barley malt

I ended up here:
http://www.freewebs.com/nq3x/beer.htm To quote the site "The Briess two-row weighed, on average, 1.5 lbs/L; the Maris Otter, 1.32 lbs/L"
From that, I garnered that a "gallon" of malt is about 5.3 lbs. So a 55 lb sack of grain when poured out will fill about 10 gallons of a container.
I bought the 18 gallon container size to give me some breathing room. I'm using fresh thick trash bags as liners in the containers, and twist tie them tightly when not in use. Fit the lid on to keep out critters, dust, etc.