I think the best method would be to glue a piece of thin plywood to the bottom of the tombstone (cut to the same size) and then using a spade type drill bit, drill 2 holes through the plywood into the tomstone and glue a length of 1/2" PVC pipe in the hole, then hammer some pieces of rebar into the ground and slide the tombstone onto them. Probably should paint or seal the plywood to protect it from moisture.