Those plastic wiring covers were for deluxe door panels with the courtesy light, and included door speaker wiring should the car be so optioned, as well as a door ajar wire in the case of a Cougar should it be so optioned.
That being said, it still begs the question what sort of wire cover would be used for standard door panels when optioned with door speakers?
If you're cloning a Shelby why aren't you using the deluxe door panels though?
On a side note, grills were flat with standard door panels instead of scalloped out with deluxe door panels.