If you are using them for structure, may I suggest shrub pots? These are the thin walled, black plastic pots shrubs, etc come in from the nursery. They are cheap, about the right size and sink with less concrete needed. I went to a local nursery, explained what I was doing, and the guy gave me several dozen. I simply put an old Wal-Mart bag around one, put it in another one, put in my structure material and pour the concrete. When is sets up, I pull the pots apart, remove the Wal-Mart bag and start over again. Good luck.
