Here's a coaster I designed. It's pretty simple. feel free to add your own designs onto the blank area.

After I printed this I used glue stick to glue some felt to the underside of it. Not sure if felt is a good choice of material for that or if glue stick will keep it there for long, but it seems to be doing fine for now.

Depending on your machine, if you try to print it flat it might warp pretty bad. I printed mine on its edge and it works much better.