Yes, you’re crazy. Stainless steel won’t last a million years. Not even close. You should go with titanium instead. That would also create a massive density difference between the two pieces in case someone lifts them up separately. Feeling the weight difference of the two pieces is very confusing for most people.
How about the bottom of Mariana tench? The intense pressure will make sure some Mr Rando can’t just pop in one day and smack it with a hammer. If you keep this relic in the remains of the exploded reactor in Chernobyl, some nut job can just run in, cause some damage and run away. Sure, they will pay with their life, but that won’t fix the hammer marks on the cube.
Chemical dangers are another option, but those kinds of places aren’t stable for a million years. Some volcanoes spew sulfur dioxide, which would be a good repellant, but those vents open and close in unexpected ways.