The issue they were dealing with was that the material would be hazardous longer than almost any known language has been spoken. We could bury the waste, civilization collapses, and then rebuilds and it would still be hazardous.

So how do you convey the danger in a way that will stick?
The messages as written are more the point to convey, rather than literal words.
The pyramids had big warnings of bad things if you break in, but combined with the decoration we were convinced something special was there.
So how do we structurally convey "this is not a place of honor”?
They had notions like covering the site with trash and so on. You don't bury a king with discarded animal bones, and you don't pile trash on something you want to honor.

I think they ultimately decided that the best path was to largely leave it unmarked, but case it in enough hardened concrete that anyone in the future would realize we weren't totally unsophisticated, and at that layer start putting more directed messages referencing chemical elements.

Some of their diagrams were pretty good though.

If you stand on a ledge, it could fall. If you fall, you die. Don't stand on the ledge.
Digging here is like standing on a ledge.

I was also really taken with the notion of genetically engineering cats to glow near radiation, and spreading the belief that glowing cats are dangerous.