hi Amy, I am trying to fill clay molds and have come upon the same problem. I talked to a concrete guy and he suggested i try Vaseline. U might also try hot glue and tape for the seams, concrete gets hot as it cures, but i think i can melt it off if it sticks. Can u cut your mold on the side than your seam will not show. I have not started my projects yet, they are small trees. would like to see if it works keep me posted. Lee Kelly
Contact this seller on ebay. http://www.ebay.com/itm/Latex-Medium-Skull-with-Runes-Mold-for-Plas... I bought an adult size mold from him and I can make plaster and cement skulls all day.
Before I forget- this is the instructions he gave me for his mold.