I use the holders for signs. Like the ones used to hold political signs. Cut then in half and you get two tombstone holders. They are strong enough to flex in the wind but stay in the ground. The thin wire doesn't tear up the stones.
I use the wire holders for political signs (coro) for my stones. I cut the wire in half so I have holders for two stones. I haven't lost a stone yet even on the windy days. The wire is flexible enough to allow some movement but they seem to always stay put. You can heat the wire up for easy insertion into the foam.