How to make a volumetric square spotlight? (projector light)


I am trying to make a model of a projector that projects a picture to the wall.
It is the first time I’m working with Blender so I’m still quite an amateur.

For the picture I just added a plane and the picture as texture.
But to make clear that it is projected by an projector I would like to make the Light visible. I already saw some tutorials onto how to make a volumetric spotlight but mine should be square and I didn’t manage that.
At the moment is looks like this:


I hope you can help me!