If you're not going to boat-tail the car, the 'end' of the diffuser will be your separation edge. Like with the VW XL1. It's inset enough from the bumper, that a pedestrian, snaking their way through a parking lot won't be apt to hit their shin on it accidentally.
And diffuser isn't going to do you much good unless the entire belly is paneled in 'upstream.'
And, if you can do the 'long, low-angle' diffuser, it will pay you higher dividends than a 'short, high-angle' diffuser.
The 2022 M-B EQXX was designed around the near-eight-inch diffuser extension, which also pinched its effective-diffuser-angle down by three-degrees, reaccelerating the flow velocity to match that of the top and sides of the car. This killed the vortex drag of the subby, high-sloped, sub-35-mph diffuser. A Cd 0.010 drag penalty.
Tuning the nose of the car with the diffuser extended allowed M-B to tailor the amount and direction of air going around the car, to achieve this non-vortex-forming 'matched' velocity at 35-mph and above.
By retracting it for low speed and parking, ground strike potential was eliminated, plus the pedestrian safety issue was resolved.