When the rice has cooled, prepare the sushi: Lay a piece of plastic wrap over a bamboo sushi mat with the long side of the mat facing you. Lay a sheet of nori over the plastic wrap. Spoon ¼ to ½ cup rice over nori and spread into a thin layer over the nori from left to right, leaving a 2-inch edge on the right side. Place 1/6th of the carrot and radish pieces 1-1/2 inches from the left, making a line of vegetables 3-4 pieces wide. Roll from left to right using the bamboo to keep the shape. Seal the roll by moistening the right edge of nori with fingers dipped in water. Cut the roll into six pieces. Repeat to make 6 rolls total. If desired, serve with soy sauce or tamari and pickled ginger.