Found this on Pinterest and thought I would share it. Not my creation, so show the content creators some love with a webclick:
How to Build a Rain Barrel. This could catch the rainwater off a greenhouse or shed.
Looks like a simple, easy to build, and inexpensive solution for storing a few gallons of water. The only thing that I would change about this would be to use a bulkhead fitting for a better, more reliable seal.