We've discussed before the elimination of the Roof vent and going to something more practical ... Here's some links to the NASA duct , an very efficient , low drag air scoop .... would make a perfect vent on TD and looks COOL too !!! Check it out for yourself ...
http.//naca.larc.nasa.gov/reports/1948/naca-rm-a8a20/ http.//naca.larc.nasa.gov/reports/1948/naca-rm-a7i30/
Sorry , Gov site is being uncooperative(typical ) .... here's some commercially available for race cars
http://www.discoveryparts.com/cgi-bin/s ... ct=ducting
try this ... http://www.recumbents.com/wisil/nacaduct/naca-duct.htm