Geometry for modeling plumes using MHD equations
 

I have just begun with Openfoam to model electric thruster plumes. The geometry for such analysis is a little confusing to me and I am looking for sources or some guidance as to how I might start building one.
Right now I am focusing on making a simple cube at the exit of the electric thruster.

Any help would be appreciated as I am a beginner in both CFD and OpenFOAM.