Place gravel. Scoop out letters in the gravel. Pour liquid cement in the grooves and over that sprinkle coloured sand. Let it dry atleast for a day. Then gently sweep off.Whatever loose sand that is scattered will be swept away. The sand stuck to the cement will highlight the letters. Good Luck.
Big rusty metal letters would look great. Maybe there is a metal works company that might do it for you. My son works for a metal works company and they have done some specific things for us. I don't know whether it's a matter of "who you know" but maybe it's worth a try. Just as thought!! Good luck with your project.