I saw a good post on this forum showing the install of Range Rover fender vents (plastic trim pieces), in a van fender. It looked clean. I remember him saying that he removed the fender and cut holes in the inner wall of the fender to help the air escape and find an easy route out of the vent.
Maybe someone can find the link/thread. I am not good at search on these forums.
2006 RB E250 Ford High top, Diesel 6.0 Bullet Proofed (ouch $) 4" Camburg Eng. Lift, Method Wheels, 33" Toyo AT, RB50 interior (homebuilt)