[Big Dave]

I think you are on the right track with a center ridge. The diagram shows a vehicle having more or less the same cross section along the longitudinal axis.

As seen in an earlier posted about a pickup truck bed canopy, the optimal shape has a ridge.

Vortices have weight. A center ridge will tend to guide the "A" pillar vortices outboard where they can simply fall off the vehicle before they interact with the roof vortex. The other option would be a fence to seapate the "A" pillar and roof vortices, but a fence will aggravate and already compromised rear visibility sitation.

Can you extend your roof ridge out to the end of the "bubble?"