A lot of coasters have holes in them and the condensation of the glass just drips through to the table top. Not of much use, then. 
This coaster is a two-part design so that when the glass condensates the water drips through the holes and into the tray underneath. 

Hardly an Earth shattering design, but it works. 

I like to print my parts By Object rather than By Layer, when possible, and this is the way I have set up the 3MF file. You can change this, of course.

PETG recommended if you're going to use with hot mugs.