I got mine from the ford dealer ship, I think it was $250 or something. It has a cutout with a clear plastic layer for the rear liscence plate, cutout for the antenna, big mustang logo on the hood, small logo on the rear. You can order for spoiler/no spoiler.
I used mine while it is IN a garage, but there are holes on the bottom that are for bungee straps to go from one side, under the car; then connect to the other side.
I personally would trust no car cover out side, although it seems secure, the wind can usually screw up alot of things, you would hate to go outside and see the car cover gone.
buy one of those poll barn type deals with just a roof for $1200
Under a cover with blowing wind you could end up with spots where the paint was literally rubbed off. My gt's previous owner had a cover on it. I can tell via the 4 spots of the bumbers where the paint is missing. Its way too windy on Maui ...... consider your climate before using one.
My car sits in the driveway with dark tint and a windshield thingy blocking out the sun. My vipertruck is in the garage. (hows that for blasphemy!) LOL