The early originals were typically silk-screened with a thick ink in WHITE for the circle, then followed with a screen in RED with the caduceus and lettering. I made a silk screen for the lettering and insignia, but cannot find it in my silk-screen carton in the shop.

I made the circle with cardstock cut to diameter and used automotive "bumper paint" in white to spray the circle. (Bumper paint stays more flexible) I made the screen template using WORD and a DIY kit like
http://ezscreenprint.com/ Then used a thinned red latex craft paint (rather than ink) to apply the silk screen image to the litter. They held up for about 4 years, then I replaced them with originals. Another guy is still using them after 5 or 6 more years, so I guess they look OK.
Late war originals (late 1944 & 1945 I believe) were black ink silk screened without the white background circle or red lettering (like the one shown above) but I preferred the early look.
No luck on my original graphics file. If I find my silk screen, I'll re-post here, but I just had spine surgery and won't be digging out in the shop for a while.