I store my bulk grain in 5-gallon buckets using Gamma Seal lids. Gamma Seals are terrific; their only negative, IMO, is the price.
As an FYI, Here's a place that has them for less, and if you could find a few people in a local homebrew club to do a group buy, you could do even better.
http://www.freckleface.com/shopsite_sc/store/html/gammaseals.html
I have them on a bunch of buckets; I lucked into a dozen at a very, very good price a while back, so it's easier to use them.
My buckets:

As an FYI, Here's a place that has them for less, and if you could find a few people in a local homebrew club to do a group buy, you could do even better.
http://www.freckleface.com/shopsite_sc/store/html/gammaseals.html
I have them on a bunch of buckets; I lucked into a dozen at a very, very good price a while back, so it's easier to use them.
My buckets:
