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 my rainbarrels are daisy-chained. My primary rainbarrel (storebought) is fed directly through a little port attached to the downspout on my garage. The overflow is attached via a tube that feeds my secondary rain barrel (homemade from a plastic 55 gallon drum). What I did here is take another plastic tube from my secondary rainbarrel's overflow and fed it through a hole on the lid of the top ice cream tub in this setup.
So for me, its an overflow barrel for my overflow barrel. Each barrel has to be slightly lower than the last so gravity can feed it from one to the other, hence why I could only stack three ice cream tubs for my setup.
If you were to make this yourself, you would want to cut a hole in the top lid large enough to feed your downspout into. Then you could add as many tubs as you wanted to get the capacity you want.