Create the shape of your urn using your polymer clay. Wait for it to dry until it's completely hard.
Use your mold release to coat the inside of the clay form and one of your Styrofoam coolers.
Mix your concrete molding rubber, following the directions written on the package.
Pour the mixed concrete molding rubber into your cooler. Push your clay form halfway into the rubber and allow the rubber to harden around it.
Draw the line of the mold along your clay form with your permanent marker. Carefully remove the clay form from the concrete rubber mold. Now take the mold out of the Styrofoam cooler.
Repeat these steps to mold the remaining half of your concrete molding rubber.