I haven't been able to find much on what materials have been used. A paper on aircraft wings said they used 30% open material for their testing. Standard window screen is about 50%....which is what I'll give a try when I find the time. It appears that the porous material is set above the skin of the object, not directly on it, creating a second air layer that also acted as a jet at the rear edge of the object, which in itself appeared to reduce drag about 13%. I am still digesting this info. Also, in contrast to Mr Wood's diagram the whole top surface and front round edge of the body was covered with the material, not just a small section of the back and the rear. I suspect covering the rear has little or no effect. A recent review (April 2014) of Vehicle drag reducing methods in The International Journal of Mechanical & Mechantronics Engineering (
http://www.ijens.org/Vol_14_I_02/145...JMME-IJENS.pdf) seem to have several methodologies aimed at breaking up the rear vortices in order to increase pressure at the rear surface of the vehicle. Interesting reading.....