but it does not matter if using a slab, cube or otherwise, right?
I haven't tried it with a slab - but one issue would be the slab has 2 materials in the geometry from memory, and you really want just one.
btw: When having th efog material put against the wall.. it goes wrong.
Yes - the rayepsilon is being ignored in this case - which looks like an Octane bug - I will report this to Otoy. So for the moment - do not have the cube touching the walls - which should be fine for your scene. I added the "scene scaling" parameter (in the Configuration) to try to get around this issue - thinking it was a scene scaling issue - but I realised now that it is not.
For working out the workflow for this, I recommend a darker interior, so you can see the effect better.
Set the focus manually - or to get things worked out - set the aperture to 0 - since it's hard to see the rays when out of focus. In your scene the autofocus is hitting the cube.
FOG material needs a scale NON-ZERO. If your scene scale is one - a scattering scale around 0.02 should give you a fog effect. Too high and it will turn the cube into a white block. Set phase to 0.75 (not 1). Roughness 0. Opacity 1.
Good luck!
Paul