Well, I can confirm that the part I listed also works. I think the only difference is how long they are. So for anyone ordering online or from the local dealer, just be aware that Ford's diagram and parts listing is WRONG.
Here is the parts breakdown of what you need.
L1MZ-7811778-A Under Cover
W714040-S300 Wheelhouse Liner Rivet (qty 2)
W715060-S450B Screw And Washer, 4.2 x 25mm (qty 4) -- or W716345-S450B, 4.2 x 20mm (per TMMST)
The two rivets and washer/bolts should cost about $10 total. The two nuts on each side of the rear under cover are already on the car. Install takes about 15 min tops. Just put some cardboard on the floor and lay on your back.