I have started with a render with the standard colorisation below Set the brush as above and brush across the area required for general volume lighting similar to that shown below. Apply Filter > Blur > Radial Blur Zoom 100%. The result should be similiar to belowusing the above eraser settings. Lightly brush over the lighting like the image below Apply Filter > Blur > Gaussian Blur radius 2.5. Using the above settings for the lasso tool while holding shift select from parts of the render as shown below. Now simply delete these unwanted areas from the volume light area. Here is an example of it in use.
