I tried a sunken area in the bottom of the saucer at one point, and I think aridas
had done so earlier in his drawings. I didn't like the way it intersected with the central fuselage - but OTOH, I didn't think to try what you did later, which was to shift the central axis of the saucer backward off of center. I may try that and see if it works better.
The color variations here are based on those from your model, adapted to the contours and style of this one at this point.