Circular Surface To Tally Over - Guidance Needed

Hi all. I want to tally the neutron current/flux in a circular, 2d area, above the reactor.

I want to be able to say at e.g., 10m above the model, the flux at that surface is x.

I have tried using a cylindrical mesh, but I can’t seem to isolate the top surface, and it is confusing me.

Any help with this would be really appreciated, I have been banging my head against the wall with this for a few days…