If you want a single layer of 0.2mm for the square at the bottom, change the height of the square to 0.2mm and it’s z-axis to -0.9.
Keep in mind, a single layer will be very translucent, it will show through the material of the disc.
If the square is white and the disc is black, the square will be a light grey. If you reverse the colours or will be a dark grey.
Using my blue and white combo, the blue will be very faint. If the square was red it would look pink. I think you get the point. You need at least three layers for close contrasting colour to look solid and 5 for very
You are really making me work for this one.