In the past, I've built mine with two pieces of PVC in the middle near the bottom. Just hollow out a notch and glue the two pieces of styrofoam together around the PVC. Then drive rebar into the ground whereever you want your tombstones and slide the PVC over the rebar. Easy peasy and no damage to the tombstone. You could retrofit a tombstone the same way, jus glue the PVC to the backside of a tombstone.