Generally this technique doesn't benefit the car that much at speeds under 60 mph. I don't recall the exact number from wind tunnel tests but its not as simple as simply cutting a hole in your fender. The design has to not add drag for it to work properly. The design that seems to work well with minimal customization is...
I don't like the look of it but I know it works okay. If you want to make it look really cool and be functional you are looking at a lot of custom work and testing. Usually messing with aerodynamics too much just hurts performance.