I store baking soda by the kitchen sink in a Progressive Prepworks Mini Prokeeper. This had a good, tight seal that keeps the baking soda fresh and dry, and has a little dusting insert for easy shaking. The 1.5 cup capacity is meant to hold the contents of a standard sized cardboard box of baking soda, but I don’t buy those.
For the past three years I have purchased 13.5 pound resealable bags of baking soda. I bought my first bag in January 2018 and found it lasted me exactly one year. I have purchased another bag each subsequent January. The bags are cheap (I pay an average of $8) and keep the bulk baking soda fresh and dry for the year. I fill the Prokeeper container from this, and otherwise keep the bag sealed.