
Certified GKH Gardening Expert
I like the idea of placing a plastic pot upside down inside your large container and then filling with soil.
Another idea is to plant inside a less expensive plastic pot that is not so deep as the one you use for the outer pot. It should fit nicely inside your outer container (make sure it has drainage holes). Then you can use your filler of choice below that pot. Then cover the top with moss to hide the inner pot if needed. You could also stand that pot on top of the inverted plastic pot instead of fillers.
Avoid using rocks, mulch or broken clay pots as they can impede drainage. Plastic jugs are sometimes used or smaller plastic pots.