I'm not sure what scale you're working in, but Gold Medal Models makes both shaded and unshaded hull numbers in 1/350 and 1/700 scales.
Pre-war, the hull numbers were quite large and shaded with black, but during wartime those numbers were not to be larger than 24 inches in height, and were unshaded. Color depended upon the actual color of the hull, white for 5-N and 5-S (dark blues) and black for 5-O and 5-H, although I have seen white numbers over 5-O ocean gray paint. Names of the ships are to be painted out.
Hope that helps.
Jeff