I purchased the MU4456-B for $99.95. I bought 4 extra twist lock tie downs so I could weave some colorful climbing prelon rope through it for a cool look. This install was easy but I am going to add some weep holes to the track to let water out and add a few more screws.
Here is my cube with roof rails installed. about $90. With the roof rails, you can pretty much install them to what width you need. I kind of had to push them in a little to use my old rack so they are not quite parallel to the edge. The roof gets more narrow towards the back