I print the numbers in reverse colors, white on black using Microsoft Word. I use canopy glue (Tester's Clear Parts Cement) to glue them to a blank plastic piece cut to size. Touch up the edges with a Sharpie so no white shows through and place them. Whenever possible, I have the numbers on the inside to protect them.
For the removable number boards on headlights, I sand/polish the old numbers off and glue the above created numbers to the plastic. A couple coats of clear coat will seal it as those number sheets are, by necessity, on the outside.
