I don't think raw materials (fence, guardrail) will do the job. It means nothing to people. But I do think a nicely worded sign will. Wasn't there a sign there at one time? How about one of those signs cast in bronze that we see along roadways, like they use for the tavern site at Sooy. It can have a footnote..."Please keep vehicles off".
Although that would look really nice, and compliment the site well, I honestly think it would be cut out and scrapped so fast. That's why you don't see that kind of sign very often in the middle of nowhere. Just sayin.