I would get a 24x24 square of 23/32" sanded plywood, and make one!
You can use a thumbtack, piece of string, and a pen to mark a 20" circle. Cut it out with a jigsaw leaving the line, and sand to the line. Add some heat-to-apply wood veneer edging, stain to taste, and lather on the polyurethane. Quick, cheap, and EXACTLY what you want.
The more power tools you have the easier it will be to make. If you have a router you can make a quick circle-cutting jig out of some 1/4" plywood to make the initial cut so that almost no sanding will be necessary.