When I run a range or conduct a class the first thing I do is pass out ECIs (Empty chamber Indicator or Bolt Open Indicators.
They are put in the rifle and kept in the rifle until the shooter is in the act of shooting.
Any one can instantly look at a rifle and tell its safe. Add to that, all rifles are grounded and shooters step back from the firing line.
I even use the ECIs on my range when I'm the only one shooting.
Certainly cheap enough. They don't distract from the 4 basic rules, but just let others see the rifle is safe.