The rotors have a negative cylinder added to the center which leaves a 55mm round by 4mm deep hole to insert cork. (I have a laser engraver, so I used LightBurn to cut a coaster down to size then engrave the images on to the cork.) The rotors thickness was also increased to accommodate for a 4mm thick cork coaster and still have a surface on the bottom. 

I also increased the size of the caliper  to accomodate the increased size of the rotors.