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So this rear diffuser will be parallel to the ground, with fins extending downwards for the "diffusing" effect you are going for? What's the purpose of this panel? Drag reduction?
To me it seems the point of these things on cars is to bring the air back upwards to minimize the wake behind the vehicle. Thus the >7 or >10 degree angles you referenced. I think the fins themselves are really for stability under cross winds, turbulence, etc. If you want to energize the boundary layer of the air flow as it leaves the panel at the rear of the car, why not just place a row of delta-wing style vortex generators? If you build the entire panel flat and then extend fins down into otherwise clean(er) airflow, and at an angle to the axis of the car (why?), you are only going to create drag.
I would suggest doing whatever it takes to put some vertical angle into the panel, from the lowest otherwise mostly smooth portion of the underbody behind the rear suspension / fuel tank, up into the lower edge of the rear bumper. Then fins parallel to the car. my 2 cents.
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