I would suggest a cookie cutter round ring of the specific size, and clay or play dough. Then fire in a kiln, let cool, and paint. Or, like in Modern Magick by Kraig, trying to make one side convex? Bowling Ball laid on top until clay hardens?
The geometric suggestion with a centerline is good for the painting imo.
If you're ,making it our of wood, I would see if you can find scrap wood that is round of the specified size and have it sliced at Home Depot. Then use the centerline technique to paint the four quarters. I'd also use Israel Regardie's The Golden Dawn to see the King and Queen color scale to paint the proper hues.
You will need to learn Hebrew if you haven't already, as Adonai ha Aretz and Sandalphon at the least need to be lettered and their seal from the rose cross be utilized to construct the sigil to paint on near their names. Ishim may also be on there. Modern Magick will help here, and I think the consecration ceremony in Modern Magick is better for solitary practitioners that dont go overboard on a temple like Golden Dawn or Thelema.