Square coaster with rounded corners that stack on top of each other.  There are two holders for the coasters, one that will hold 4 coasters and one that will hold 8 coasters.  

The coasters will need to be printed with supports around the outer edge, turning on Support on build plate only is sufficient.  The holder does not require any supports.  

I suggest  printing the holder at 0.15 mm layer height.

All other defaults should be fine or tweak as needed.

You can adjust the top fill pattern to give each coaster a bit more style, embed your own text, or an SVG file / logo.  In the examples photos I did a layer height color change.