I just use the Homer buckets from Home Depot, I keep the pellets in the bag and twist it up with a wire tie and put them in the buckets with lids on.
You can get o-ring seals for these buckets if you really think you need the extra sealing but mine are indoors so not that worried.
Also I removed the grate in the hopper so I can move pellets around easily and I use a cool perforated scoop to fill and remove the pellets from the hopper, it's easier that the back door thing for draining the pellets. Plus you can sift out the dust when adding pellets to the hopper.
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