As I recall, Ond did something kind of similar.
Take some sheet plastic, cut a rough circle from it, then bolt it into a drill press centered and use sandpaper to get the edge clean and finish it out completely circular.
Finally, drill through the center with a 1/2 bit to open it up for the stick.
It'll be completely flat though.. While it's in the drill press you might exp with using a chisel or carving knife on it while it's spinning to etch out a ring or two on it to dress it up a little.