I have a store bought Styrofoam tombstone that is one inch thick. I plan to cut a piece of one inch pink foam board to glue on the back to make it thicker and stronger. My question is this. I want to use something on the sides of the tombstone where the Styrofoam and pink foam board meet to both seal it and make it look like it is one piece. What type of material could I use? I'm thinking about something the consistency of spackle or wood putty. What would stick and not crack? Any ideas?