the cougars and mustangs used different dashes. The mustangs had the "scooped" inserts, while the cougars had a straight pad on them. If you swapped the whole dash frame and everything, it should work, and you will have something unique.
If you already have the frame and just need the cap, Mustangs Unlimited sells the cap for the Cougars. They also have a free Cougar catalog that I recommend.