Personally, I think you make it as dangerous as possible. So rather than shielding the waste, make it deadly in a matter of hours or minutes.
This won't prevent people from trying to extract the contents, but it should prevent them for succeeding. I think a few dead grave robbers / explorers is preferable to the waste being extracted and sold back to civilization.
I was thinking more along the lines of burying it deep under an inactive seabed. We’d have a hard time now digging things up. We aren’t even able to do simple floor mining well.
Once people have technology back, then presumably they’d be able to detect radiation and avoid the contaminated location?
In 100000 years the biggest threat is probably glaciation submerging very large areas of Finland.