I have a cover for my 97 C280. I use it mainly to keep the car clean, not really to discourage theft. My cover is the dealer item and is an exact fit for the car, it even has a little pouch for the hood star.

I do use a lock and cable that go under the car and attach each side of the cover to each other. Anyone that wanted could just lift up the front or back of the cover and have a go at my car. However, if someone wanted to steal my cover, they would have to cut around the lock area, this way no one can just lift off my cover and take it. They have to have a sharp knive or a pair of scissors if they want to keep the cover.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record