All you need is the Principled Volume shader.
- Create a volumetric cube
- Give the volume a very light color, and use Density to control it
- Use real units for light intensity (lumens). Related answer: Simulating a 40W Light Bulb.
Lights: 10 million lumens (a really strong searchlight), Density: 0.1, Samples: 2000 + Denoiser