[j-c-c]

My diffuser application is primarily seeking downforce. It is usually thought a proper diffuser design/installation usually compliments a proper rear wing. In this pic car has mini spoiler, with rear trunk mounted oil/trans fan cooled exhaust vents, the two still foamed diffuser slots are with high temp epoxy material that will have shaped exhaust outlets. The center upper open area is for tag mounting and the exhaust of the rear end fan cooler. Length is nearly to rear axle housing and taper much lower than 7%. Its all CF/Kelvar construction, 1972 Dart.

edit: Something included in my above DF solution, and seldom mentioned with diffusers, there is allowed and intended an amount of "waste/bleed" airflow to had above the diffuser, since it is extremely difficult to efficiently seal all air intrusion above the diffuser, and to gain the additional benefit to be found by extra heat extraction here from the very nearby exhaust plumbing that might degrade the composite diffuser with long exposure.