I found an interesting link with instructions for making a ferrocement composter. The instructions show how the mold is made and instructions for making either a tie together unit or panels that can be ferrocemented together once the panels are complete. The design is a tidy little composter that will last forever.
This is very much the same technique I want to use in building the walls of my house. The technique as described doesn’t use as many layers of reinforcing, but the idea is the same.