I can only think of one way to make the sandwich if the Dresden Plate has the finished pointed end blade. Sew the backing circle right side in to the batt, trim the excess batt close to the stitching line, cut a slit in the center of the batt and turn it right side out. Iron the edges until it is perfect and mend the slit in the batt with a piece of batting tape. You should now have a perfect circle to either machine sew to the Dresden Plate or hand sew.
I thought of birthing it through the center hole but you would lose the pointed ends.