the most simple solution imho would be to have a setup somewhat like this
a low drag shape to start with (this car had a 0.28 Cd ) but it could be made to acts as a duct from the engine bay to the mirrors, so you could vent engine bay air trough the mirrors. or you could have an intake at the base of the windshield. the first solution would have the added benefit of venting warm air alongside the mirrors and the side windows wich would be nice in winter, but it might also increase airflow throuh the radiator ever so slightly, allowing it to be ever so slightly smaller.
none of this might be worth the investement, but on the other hand i don't think there's any downside to this setup and all it needs is a simple duct, no high tec involved. plus the way i see it all these little "neglectable" benefits might add up with other clever tweaks and nock another 0.01 of the Cd