I would just check with Ford and see what kind of deal you can get on their snap-on cover. That is what I have on mine.
I would take a pic, but mine broke (they said it is the first one they have seen break) over the week-end. One of the plastic corner pieces broke off somehow. It is covered under my warranty so they are ordering me a new one. The only water that has ever made it in has been from around the tailgate, and could be prevented by putting in some type of tailgate waterproofing strips
They are basically like that one but the middle cross-pieces (there are 3) are flat instead of round. It is also crafted out of aircraft aluminum if I am not mistaken so it is really light.
You can see a little of it in this pic: (It does not hang over in the rear, it was unsnapped)