The last time I checked, dry ice "melted" just as unevenly as the wet stuff.
Keep in mind that the main problem with the project is that the hole is the exact same size as the slab, so the slab has to be lowered down with the edges of the slab perfectly parallel to the sides of the hole.
If the ice, wet or dry, melts faster on one end (or side) than on the other, that end (or side) is going to go in at an angle and the slab will become wedged part way down into the hole.
One of 2 things would have to exist for ice (wet or dry) to work:
Either:
Your ice *and* your slab would have to be perfectly flat and of perfectly even density so that the ice melted perfectly evenly...
or
Any variation in the density of the slab that caused it to be heavier in some spots than in others would have to compensated for in the density/height of the ice. Since pressure creates heat, a heavier portion of the slab is going to cause the ice to melt faster in that area than in others.
Good luck in either of those situations.