A diffuser can be as simple as a plate in front of the front wheels and another in front of the rear wheels. You'll see that on many cars built these days. If you look at Mercedes, BMW's and Audi's they have almost no rear wing or spoiler. Depending on the model they are all capable of hitting 155 mph and the cars stay stable at those speeds due the aerodynamics under the car. What is nice about that set up there is no rear spoiler dragging when your going straight. I can't use the same set up they used because of the underside of my car but there are different methods. I intend to also put a front dam with side skirts made with a product that will flex if needed. That will keep the pressure under the car lower than what it is now so that will improve it. Although it is better to have a smooth surface underneath to lower the pressure is still beneficial in handling and fuel mileage. The rear of the car has almost the right angle for a rear deffuser to work. As far as not having enough speed to have any aerodynamic effect I'm sure you have noticed putting your hand out the window doing highway speeds the difference of having your hand flat into the wind compared to slicing your hand through the wind. Everything helps so I don't see it as a waste of time or money. I've removed the chrome trim around the front windsheild and raised the ledge the window sits on the make the window almost flush with the surrounding body. I used rubber around the window which sits flush on the window and body so I've reduced the wind noise and drag. That cost me almost nothing and not much time welding the pieces in.
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