Slicer settings are important! If you use default settings this will be a functional but boring coaster.

Layers and perimeters:

Perimeters: 7

The outer ring is about 2.8mm thick, divided by 0.4mm nozzle width equals 7 perimeters. This is unusually high but it forces the slicer to work the way I want: the outer ring remains solid while the large flat area becomes infill.

Solid layers: Top: 0, Bottom: 8

This is how to expose the infill at the top. I decided to use 8 layers of infill and 8 solid layers. The flat area is 3.2mm thick, divided by 0.2mm layer height equals 16 layers (8 infill, 8 solid).

Fill density / fill pattern: whatever you like! Try different combinations of pattern and density :)