I've had great success with covers made by California Car Covers. They have multiple different types made from different materials based on how and where you store your car. The covers are all custom made and fit like a glove. Very high quality.
Custom Car Covers | California Car Cover Co. Official Site
2016 Ecoboost Mustang
2005 Mustang V6 (Sold)
2003 Terminator (Sold)
2002 Mustang GT (Sold)