Thinking further on the subject of buckets, it occurs to me that some brands of cat litter are available in large, heavy-duty, square buckets with resealable lid (but with no rubber gasket. The lack of gasket could probably be remedied by using a contractor bag as a liner, and I'd probably do this anyway for a cache). These store side by side and stack with minimal wasted space in between. I have about a half-dozen I've employed for various household duties and another dozen stacked up empty in our storage area. Now I know what to do with the empty ones!