It's not a plane, so we need to be careful with the terms. If you did the simulation in Fluent I advise post processing there: we can imprint a face too which makes things easier. In CFD Post I think Insert>Location>User Surface will do the job. You'll need to create the surface in CAD and then import: check the formats as I know Fluent needs faceted geometry (stl or mesh).
As a more general comment. When working out what you want to model, sketch out the domain and add all information you need to add to the model to make it work. This also highlights stuff you don't know. Before moving to the computer you also sketch where/what images you want to answer whatever question the model is intended to answer. In this case, splitting the volume in CAD and passing the surface over with an interior boundary type would mean it was ready for post processing in the solver/CFD Post.
The added bonus of the above is you have a record of all models, inputs etc to discuss with your supervisor/manager/customer (yes, even after 20+ years I use this approach too) and provide a record of what was agreed for the model.
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