I ordered a custom fitted one. It was reasonably priced and works great, no gaps and rolls up nicely.
http://www.heatshieldstore.com/
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My girl: Heidi - 2008 335i Sedan, Steptronic, TiAg, Black Dakota, Sport/Premuim, Aluminum Trim, Navigation, Comfort Access, HD Radio, iPod, PDC, Sunshades.... she's loaded.