I did this repair to my 67 a month or so ago. Some (not all) of the Mustang vendors sell a trunk floor (RH and LH seperate) repair panel. They are not a direct fit as the cougars are longer in the rear than a Mustang. The drop offs on a Mustang are shorter so what I did was bought the Mustang trunk floor piece and the Cougar drop offs. I only used part of the Mustang piece but now that its done it looks pretty close to factory. Looks WAY better than if you just use sheet metal. I bought my trunk floors from Tony Branda and the drop off panels from Mustangs Unlimited. The trunk floor panels were VERY thin material (not very good quality) but the drop off parts from Must Unlimited were very thick and good quality. I would call Must Unl and see if they have the trunk floor also cause it may be of thicker material.
Scott