Create Probing Surface then Surface Integrate over it

Good evening Ladies and Gentlemen!

I'm working with a 3D fluid flow model, and I want to find the mass flow rate through an arbitrary cross section of the flow. How can I create a surface that can then be integrated across to get mdot = rho*velocity*area? I am easily able to integrate with the "Derived Values" --> "Integration" --> "Surface Integration" command, but I do not know how to create a selectable surface at any arbitrary cross section!

Thanks,

TEB