Here is my point of view (I think that is similar to LDVH idea). Considering that:
1) The simulation of the cooling ducts would allow much more design freedom (and most beatifuland different cars)
2)
It is impossible to completely simulate the internal flow without simplifications and assumptions
3) A radiator template is necessary to avoid not realistic design of the radiotor itself (too small beacause too efficient or with an irregular shape, impossible to be built)
I would use a radiator template
(in RED) that must be included in the carbody solid, defining two surfaces (internal to the sidepod, but still external to the CAD model) where to measure the integral of the pressure over the surface. The best way to simulate it would be to use a "porous" definition for the volume between these surfaces, but a simplified version of the simulation could include a
fictional hole (diameter decided by the rules) that would generate the right pressure drop and where to directly measure the air flow (without passing through the computation of dp).