| My car has HKS intercoolers. In order to take advantage of their big face(s), I built ducts to force incoming air smoothly through the entire surface. The ducts are made of home siding vinyl, riveted to the mouths of sliced (shortened and expanded) stock ducts. I used the front of the stock ducts because my turn signals and foglights are still in place. So the only air getting to the intercoolers is coming through the stock facia's gills.
Siding vinyl, which I used for the front ducts:
But this post is not about the front ducts... I built them several years ago, and wish I had taken photos of how they came together. But the process involved way too much test-fitting, re-measuring, trimming, re-shaping, test-fitting again, etc. to be useful as a how-to guide. Shortly after I built mine, I helped Ashley (TT XTZ) build a duct for her driver's side HKS intercooler, and she attempted to document the process with photos, measurements, and templates (figuring the passenger's side would be the mirror image of the driver's side). But there's so much customization involved to get everything to fit together snuggly (fitting around the turn signals, for example), that I think she ultimately decided to go ductless.
Anyway, this weekend, I put some more of Home Depot's vinyl supplies to good use. In this case, I used a single strip of "3' Snap-In Gutter Filter" (total cost = $2.59) to finish the ducting, by venting the back of the intercoolers into the wheel well.
Since my car is a '93, it has the NACA-style duct in front of the wheel well, to suck air out from behind the intercooler. But those ducts are awfully small. I've been reluctant to "hack" into my fender liners to add vents, hoping to find empirical evidence that -- despite the wheel well being an area of high pressure -- venting into it actually drops intake temperatures.
Intuitively, it seems the airflow from the front would be sufficient to overcome the resistance from behind. I know that Kory (kkrofft (OH)) was doing some experiments with a thermo-couple, but I couldn't find anyone comparing an intercooler's efficiency BEFORE ducting to the wheel well versus AFTER. Even among the Audi and VW tuners, many of whom perform this mod, the prevailing sentiment seems to be "I don't know how much it helps, but it's so cheap and easy, you might as well."
I figure, if Audi vents to the wheel well with their S4 (as DSM used to with their turbo Eclipses and Talons), they must know something... So I've joined the vented crowd.
Without further adieu, the process in photos:
The "vent" uses a gutter filter made of vinyl, with a nylon screen glued to it on one side. I bought one 3' long piece from Home Depot. They sell it in the aisle for gutters and siding materials.
It has a groove running along one edge, which comes in handy for sandwiching the fender liner.
I cut it to a 14" length.
It comes with the screen already glued in place, but I reinforced that attachment with hot glue.
...And trimmed an inch of the groove off of each end, so the bottom "flap" would be able to poke through the hole sliced into the fender liner.
After measuring a few times, I sliced a hole measuring 5" x 12" into the liner. This is about two inches smaller, both horizontally and vertically, than the strip of the filter/vent material. (You'll notice I reinforced the NACA duct while I was at it. The bottoms of those tend to get scraped away over time.)
The side of the filter with the screen attached goes toward the front of the car, so the breeze will be blowing the screen back against the vinyl. It snaps into place with the fender liner secured between the ridge on the filter's bottom.
I used 5/32" x 3/8" alluminum rivets along the bottom, and 1/8" x 1/8" rivets around the rest of the perimeter, with backing plates all around.
All alluminum, vinyl and nylon, so it should hold up to the elements.
Here's the driver's side, installed. I had already replaced my radiator overflow tank with a jug from Jegs, and got rid of the carbon canister and other plumbing behind that intercooler. So the airflow has a clear path, straight through.
...And the passenger's side, also a clear route for airflow. 
I suppose they could be painted black, or sprayed with undercoating if somebody doesn't like seeing brown in their felder liners. But with the wheels on, it's pretty dark in there anyway.
My next track day will be in early November (GripFest at NCCAR - sign up!). My club has a multi-day cruise before that, and a show (Octoberfest - sign up!) two weekends from now, so hopefully the form:function ratio is good for both.
 
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How To: Cheap (but hopefully effective) intercooler vents - 
