For round containers, try using heavy nursery pots. Keep an eye out at garage sales and flea markets. Leave the bottom smooth. Never pick up an uncured container by the sides. The slower it cures, the stronger the container.
Several weeks curing and rinsing is a minimum.
A slurry of cement and water can be smoothed over the surface for a more finished look. To strengthen the mixture for larger containers, add a large handful of Fibermesh to each gallon of dry mixture. When you brush off the outside of the container after you take it out of the mold, these fibers will make it look fuzzy. Don't worry about them at this stage. After it has cured for a few weeks, the fuzz can be burned off.