Can't get transparency right for God Rays I want to get a Hologram effect like in the image uploaded here:

I need to place the hologram over a real video clip, so lots of transparency. Works perfectly, following this guy's advice here.
Now I like to have some God Rays to it (realistic hologram needs a projection light cone). High-energy-spot with volumetric scattering (material node of a cone just outside the spot light cone).
So far, so good.
Now: whatever I try, I do not get the light rays' background transparent. Black works. Looks nice

But I need to place the whole thing on top of a real video clip and I do not get some black streaks out of the light rays - however I try and adjust any nodes with whatever Alpha effect.
And to make it worse, I had it all, five years ago, and just can't remember how I did it...
Could anyone just get me God Rays with a perfectly transparent background for me; to place them onto a video clip?
Holograms are essentially like reflections, which are purely additive light without any occlusion.
Try using the set alpha command to your image with zeros occlusion set in the alpha buffer. The RGB will then end up becoming a pure emission when composited with an Alpha Over.
Note that using proper camera encodings will take the effect even further along.
Good luck.