I used a metal dryer fitting 5" reduced to 4"(drilled a small hole under the air valance and bolted it in place, large washers) and metal flex dryer duct inside the fender well up to the stock air box. I sealed off the oval fender opening to the flex duct with sheet metal and self tapping screws.
In cooler air, at sea level it was worth a solid tenth and 1mph(Miramichi New Brunswick).
In Phoenix Az., with blazing hot track/pavement and crappy air, it was worth nothing. Zip.
I did scoop up a flattened pepsi can tho...
Back in the day, Bob Cosby ran a like setup. I don't think anyone can dispute the success he had. Al Renkel also used one to go stock motored 12's on a budget, which MM&FF covered so well.
Both these guys ran at sea level, and in good air.
I would not run one on the street, too much crap gets pulled in... it was a 5 minuit job to bolt screw it in place at the track.
Hope that helps.