I think at the end of the day, the best we can hopefully for is that only one or two people grab the valuables inside, then everyone else realizes the curse is no joke and tosses the crate back into the ground.
So, the solution is to bury a small piece of unshielded highly radioactive material at the entrance, just enough to lethally irradiate the first party that breaks in? ~
If I were really trying to do a good job of it, I’d try to come up with the way to mix the waste into a hard brick too large and heavy for one person to carry, but cart-able for two or three. Perhaps i’d even use a mold to make it look deliberately manufactured to discourage breaking it apart. This way, hopefully, the waste wouldn’t be scattered in small pieces before the community realized what was happening. Hopefully, then, whatever party came into possession of the brick would #1 very visibly possess it and #2 very quickly show signs of illness. Alongside all the other safety measures, I’d hope this would very quickly tell the community that obtained it how dangerous it was while minimizing the scope of harm it ended up causing. I don’t personally see how anything better can be achieved here if we’re assuming this culture has lost all traces of nuclear physics and can’t understand any languages we have, and disregards any pictographic warnings we put on it.