Create your own coasters that will change collor when heat is applied.
Step 1: Draw a Shape.
Any shape you want your coatser to be. You can use for example Sketchup, probably one of the easiest drawing programm which you can download for free.
Step 2: Give Your Form a Thickness.
By using the Push/Pull tool in Sketchup. Any thickness you want. I used 0.5 mm.
(Thicker = more plastic = more expensive)
Step 3: Let the Printer Do the Work.
Print your coaster in thermoplastic PLA or ABS.
The plastic I used was black, but it prints it clear/white because of the heat of the printer.