Credit to GPPK for the original design.
I found this AWESOME design for d20 drink coaster so I decided to edit it a bit in case anyone wanted to use the core design. There are two designs: the first one is basically the inverse of the original design - if you want to integrate it into a print, make it a hole and insert it into whatever your solid is to get the original design right on it. The second is almost identical to the original design, but it's in a square if you so desire that.
Again, HUGE credit to GPPK's original d20 coaster design. Check it out! I, for one, will definitely be owning some d20 coasters in the near future (as of this original post). In fact, click this: https://www.thingiverse.com/thing:2576855