There are real graves and tombstones on the island, so they may used something that was already there.
Han said Luke went in search of the first Jedi temple. I’m pretty sure he found it.
On that note, it was very strange seeing a stone crucifix in one of the shots leading up to the reveal of Luke. I get that they wanted to portray the island as accurately as possible, but that was one thing that screamed ‘Earth History’ to me.