Mold a two-inch layer of polymer clay into the bottom of the plastic box and form walls up the side of the plastic box that measure four inches high and two inches thick.
Spray the object that you want stamped into the clay with mold release agent if you have chosen one. Be sure to cover the entire surface of the object. The mold release will form a seal over the object to separate the object from sticking to the clay for a smooth and clear impression left in the polymer clay.
Press the object you have chosen for a design into the bed of clay. Press the object an inch deep into the clay. You can also create your own design by using your fingers or multiple objects for a unique design.
Spray the inside of the plastic box where you have molded the polymer clay with mold release agent.
Mix and pour the silicone into the box on top of the polymer clay, one inch thick. Begin in a corner and apply evenly over the entire surface. Tap or gently shake the sides of the plastic box to level the mixture and remove any air bubbles that may have formed.
Allow the silicone to dry completely and remove the created mold from the plastic box. Press into the concrete surface.