My car is a 17’ which all have the track pack spoiler. Not sure on the material but I’m here is a copy and paste from an oem ad:
OEM NEW 15-17 Ford Mustang Shelby GT350 Logo Car Cover Covercraft Weathershield - DOES NOT FIT GT350R
Custom Patterned for your vehicle - Protection pockets for...
Came with my 17', brand new unopened. Looking for $250 USD shipped from Detroit (greater 48).
Untitled by gt35023, on Flickr
Untitled by gt35023, on Flickr