Not sure what is hard about this? its a solved problem. Mounting a human skull has been an effectve sign for stay away for 10k years already. It wont change in 10k years.
As humans, we've opened more sinister stuff than that, way before having any clue about nuclear energy. No reason why our post-apocalyptic brethren won't do the same
It's more nuanced than that. Mounting a human skull could also mean: here's a graveyard where people have been burried with their most prized possessions. There's a really interesting documentary about Onkalo, a Finnish nuclear waste repository where they talk a bit about this. I think the conclusion they came to was make it look as boring as possible. Or at least more boring than the hassle to get in :-)
Perhaps boring and desolate, Though IDK how you could guarantee desolation over such a period of time. Hassle to get in by definition makes it interesting. It would probably only deter the same people that a skull would with muchless effort than razing the area though why not combine both?
Robbers, raiders, and treasure hunters would still go in.
The people that ignore the skulls would ignore anything anyway. Robbers, raiders and treasure hunters by definition risk their lives to gain money or power. Any warning must be assumed to be a trick if you are in that life.
