The secondary ribbon cable is probably relatively short, which constrains the design of a fixture. It's possible to use it as is, like I did, but getting a longer cable will make things easier. You need to measure the width and search online for the same cable. You shouldn't need anything more than 5 or 6 inches.
Note: It would probably be best to hold off on the rest of the work until you get the new cable to help eliminate mistakes and make sure it will all come together when finished.