On most of my kits I just use a large red permanent marker for the cross and black marker for the words "First Aid". I even do this on the one gallon ziplock bags I keep in our vehicles for first-aid kits.

I have some smaller cloth kits that I would like to look a little more professional. The best I have come up with is the Boy Scout First-Aid merit badge. It is small enough to easily fit my personal kits. Two could be used on larger bags.



It is not the ideal "red cross" but it does get the point across. They are inexpensive and can be purchased at any Boy Scout store.

Good luck, TR