Just make the carved base a bit deeper where the lowest parts of the stone will sit.The base is a little awkward for carving a daiza - I'd appreciate seeing design suggestions if anyone has a similar stone in a daiza. The coins underneath are propping it up in the position I think it will need to sit in.
This one is petrified wood from Western Queensland. I thought it looked like a good mountain stone.
My first attempt at making a daiza for it. I used the router to progressively cut the depression deeper a little at a time, noting where the rock rested and taking a little more out there until it sat properly.