The SL1 already has a front air dam which deflects air upward into the radiator. The resulting vacuum created behind it probably helps draw the air through the radiator. However, this air dam is only about 2/3 of the width of the car, and a lot of air can spill around it, where it has to flow through the suspension.
I started by making a cardboard template to figure out exactly the size and shape that I would need:
After reading a lot of other modders' threads on air dams, I took a trip to Home Depot to find some suitable material. I didn't like any of the lawn edging I found, but then spotted something really interesting. It's called a KwicKan, and is basically a 48"x30"x1/16" sheet of tough recycled black plastic (polypropylene?) with some holes cut into it for handles. The idea is that you roll it up, insert it into a garbage bag, and let it go. It expands to turn the bag into a fairly rigid can. When it's full (e.g. of leaves), you pull the sheet out. It seemed to the be the perfect material for my planned mods, so I bought one ($12).
Using a yellow paint marker, I traced around my template, and cut out two copies of it, one the mirror image of the other (the KwicKan material is textured and printed on one side, and smooth on the other):
To fasten the air dam extensions to the fender, I made brackets out of 1/16" aluminum, cut into to 1"x2" pieces, and bent so that the air dam mounting surfaces would be vertical:
I used stainless steel 10-24 bolts, finishing washers, flat washers, and lock nuts, both to fasten the brackets to the fender, and to fasten the air dam extensions to the brackets and existing air dam:
Here's how it turned out:
I was careful to make both pieces exactly the same (except mirrored) so that they can be swapped left to right if I ever want the textured side outward (I'd have to paint them of course).