Roof rack will definitely make the drag even worse, not sure about the deflectors though, they may actually improve the flow.
What about a classic aerodynamic component - a spoiler? There are few options available for Soul, like these
for example. They look aerodynamic, but from various tests that I've seen in automotive magazines, some spoilers may decrease the downforce and make the drag even worse. That's why I would trust some information based on the actual wind tunnel tests.