I keep 5 bulk base malts. I bought the standard large plastic trash cans on sale for about $10 each. I found some 4-mil hdpe barrel liners ($2/ea) which I line the cans with. For a seal, I just twist up the top of the liner and tuck it between the can and grain. I think they will store over 2 sacks in each.
When one gets low, I pull the liner and grain out, put in a new liner for the fresh grain, fill and put the old grain in its original liner on top. This way, I can rotate out the older grain.