The Japanese garden pagoda I built by using plastic plant pots and cement.
I collected plastic pots of different shapes and sizes, cut the bottoms off, slit them up one side and taped the slit back together. filled them with concrete and then removed the pot by removing the tape after they dried 24 hours. They are still a bit damp so you can do "wet carving" on them to make your forms more rounded or smoothe. I used rebar again as I had with the Polatems in my earlier post, or