I would have probably used 1/8 inch aluminum. figure out the angles that you would like the spoiler at and cut the aluminum to match and draw the angles on the aluminum, drill the holes you would use to mount on both sides and then bend the alum. on your marks and mount to the spoiler and to the car and done. can still paint the aluminum and screws to match the car or leave it. I would use stainless steel allen wrench srews or something that wont rust.
Sounds like quite the project, I use to have a Sunfire and like that spoiler and Post pics when you are finished.
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