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.
So this is 3 of these stacked up together. Each is 11.4 liters. All that equates to about 9 gallons. Thats enough to water my gardens probably 1.5 times. I have a lot of garden beds though. If you just had a small one you could do more.
My total setup is 116 gallons (but thats with two large rainbarrels too) which lasts me a long time.
A good rainstorm would fill this setup very quickly.