This may not be useful to you at this time, but there are a number of businesses – ice cream shops, dry goods stores, restaurants etc – who get products in 1 gallon or 5 gallon, air tight plastic buckets, and after they remove the contents, they trash the containers. If you ask, most will be happy to give you the buckets. You can pack them with bagged foods, add a dry pack, then if you want, seal the edges around the lid with silicone caulk to make doubly sure that the contents stay dry even if flooded.
Sorry that you killed the snakes, though – I usually slide a shovel or something gently under them and take them to the creek bank a few hundred yards behind the house. I did have to kill one, years ago, because my dogs were barking at it and i was afraid it might – understandably – bite one of them. Felt bad about it, but that was a case of necessity.