I used sheet metal screws for the cover I made for the yellow car. They've never caused me any problems. No rust or anything in that area.
I think I have a spare tip you might be interested in. I pulled a full borla exhaust last year from a car and one tip has a rub through the carbon fiber can with the metal ends still being good, the other tip seems to be fully intact. If you're interested I could take a pic. shipping would only be from Mass.