Got a little leftover sand or gravel? How about extra bio-media of some kind? What about some PVC connectors just lying around?
Rather than throw away or recycle those plastic food containers that you’ve emptied, wash them out and reuse them. I love the clear plastic variety because they’re transparent, they’re often the perfect sizes, they have lids, and they’re versatile. I use them all of the time and routinely find different storage uses. You can even fashion air-driven or powerhead/pump-driven filters out of them.