You can seal the mold box with silicone caulk or clay. Clay can sometimes be preferred because it is not as messy and it is re-usable. Always try to seal the box on the inside not on the outside. Liquid rubber develops a lot of hydraulic pressure and will easily push thru the silicone or clay on the outside of the box. On the inside, it simply pushes the silicone or clay tighter into the corners.
Here is tutorial for uses of clay that will explain more: