Got these old ice cream tubs from a local ice cream place. $1 for a dozen. I wanted to increase my rainwater storage (currently have 2 rainbarrels). Realized I could stack these guys up as much as I needed. A few drilled holes and a spout off a cracked kombucha kit and bam. Homemade rainbarrel.
This is part of a set of daisey-chained barrels so I had to keep it to 3 tubs for height reasons. All told it only adds about 8 more gallons of storage but every bit helps. You could stack as many as you wanted though, within reason.
I kept it simple but you could also add additional sealing between the bottom-lid connection to limit loss that way. I will add a few extra pictures in the comments.
Additional pictures so you can see what I did. Its dead simple though.
One thing I forgot to add! You need at least some waterproof caulking where the lid meets the bottom of the next barrel to keep it sealed! Very important step.