Create a mold container out of rectangular-shaped acrylic scrap pieces. Put the object on its side on a table and arrange four rectangular acrylic pieces into a rectangular shape around the object you wish to cast. Put the fifth acrylic piece under the object. Arrange all the pieces so they fit snugly around the object, leaving only about a quarter-inch space around it.
Glue all of the acrylic scrap pieces together with a hot glue gun. Allow the container to completely dry.
Press non-hardening clay on the bottom of the container about half as thick as the object you are going to cast. Be certain to press out any bubbles or holes in the clay.
Press the object into the clay, taking care not to press it all the way to the bottom of the acrylic container.
Press eight holes about a ½-inch deep in the clay all around the object using a pencil eraser.
Pour uncooked rice over the object and clay in the container until the object is completely covered. Pour the rice into a measuring cup and take note of the number of ounces. Mix that exact amount of urethane RTV molding rubber and be certain to follow the directions on the container.
Spray everything inside the container with mold-release spray.
Transfer the urethane by pouring it slowly into the corner of the container. Allow to set for a day.
Remove the bottom acrylic scrap piece off using a knife.
Flip the container over and remove clay using a knife.
Spray the entire inside of the acrylic container with mold-release spray.
Mix the exact same amount of RTV as before and pour it very slowly into the corner of the acrylic container. Allow to set for a day.
Remove container's sides with a knife. Pull the mold pieces apart and remove the object.
Sandwich the mold between two pieces of wood. Hold together with a few rubber bands. The mold is ready for you to pour casting material into it.