Roundels are so very simple that I'd do them with UltraCote or Monokote. Use a protractor with a center point to size each piece for cutting out, and the hole that leaves will give you a way to accurately center the pieces.

Place the big background piece and iron it down. You should still be able to see the center pinhole from the protrtor. Push a straight pin through the center pinhole of the next layer and stick the pin point into the big pieces center hole. Slide the layer down and start ironing it from the center out, etc