In recent test I read (which was well measured with probes etc) so pretty scientific. Came out with the Maxogen coming top.
As you say it's the location which is challenging especially as they are not small..
If you run a windscreen all the time then there is a high density area of air at the base of the windscreen therefore you could tap into this area through a grill at the rear of the engine bay. You may need to fabricate ducting/airbox to seal onto the under-side of the bonnet... alternatively manufacture a duct that taps into the air under the front wheelarch... it obviously needs to be cleaned but it is an area of high pressure.
