What's the right fairing size?

I was gifted a set of Thule AeroBlades and decided to get a fairing to cut down on the noise.

The guy at REI recommended I get a 38" AirScreen, but I installed it and there's a gap between the side rails and the edges of the fairing.

Is a fairing supposed to span the entire width of the roof? The next size up is 44" but I'm concerned that would be wider than the side rails and stick out.

If anyone here has a fairing on their roof, how wide is it?