IMHO the rectangular opening is Okay, it needs to conform to the internal ducting to the rad/heat exchangers. An oval opening would need to transition to their rectangular shape. Radiusing the edge OTOH could be important to create a rectangular bellmouth.
If you do the air curtains, maybe a wind tunnel will find you.
There is some internal structure involved. The exit is a tall narrow slot that vents across the face of the tire, not onto the tread. And I'd move the outer edge of the intake opening forward to make the openings 'cross-eyed', increasing the apparent opening to the sideways-moving air. Intake larger than the exit as it's a convergent duct.
Here's a picture that I hadn't shown because I wasn't happy with it (and there're problems trying to reproduce it):
It's basically a lipped vertical air dam curved to match the bumper, with flat-faced wings to scoop the air. The shape of the junction of the dam and wings is not right; I struggle with the design tool.
