What your diagram is showing is just basically what a flat bottom diffuser does. It isn't creating downforce, it's just straightening air and eliminating drag. What a downforce derrived diffuser is designed to do is bring the top and the bottom of the cars airflow together more amicably than just shooting the air out the top and the bottom. Air traveling over the top of the car goes faster than the bottom of the car, essentially creating Bernoulli's Principles of Lift over the entire car. If you look at a side profile of a car, it is essentially wing shaped. The air going over the top of the car accelerates faster to meet the slower moving air under the car, which effectively pulls the car off the ground at a high enough speed. For years, car manufacturers have used wings, lips, splitters, etc to counteract this effect, and it works. But it creates drag, slowing the car down, lowering its top speed, etc. An even better thing to do is invert the profile, causing the air under the car to move faster than the air over the top of it, sucking the car to the ground rather than pulling it into the sky.You must be getting ideas from other cars like this one for example
Maybe it can be functional?