[AtomicLabMonkey]

You might need to do an actual sealed air box from opening to radiator if nothing else is working. Splitter would be my next thought, which has an obvious downforce bonus for handling and they're fairly easy to make. Increasing airflow through the rad is all about pressure differential, need to increase P in front of it and lower P behind it. Either one or both of those options costs way less than $4k to fabricate.

If the coolant temp sensor the ECU uses lines up with your gauge then that's good enough correlation, readings should be accurate.