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The ingredients are always the same, however: stones, Rockite cement mix (don't use anything else), cement sealer, and a three-sided mold. Chen constructs his molds from wood lined with aluminum flashing — which gives the bookends a smooth surface texture — then seals the seams with Plasticine to keep them watertight. "You could probably use a plastic bin that had relatively sharp corners," he says. "That would probably work too, and you might not even need the flashing."