Help designing a diffuser

[d_dan]

Hi everyone,

Was hoping someone would be able to help.
I'm looking to make a functioning rear diffuser any tips, links or info would be great.

Here is what iv found from my info hunt online.
The diffuser would want to start at a the rear axle.
The fin's are to be parallel to the road.
the angle of the fin's are to be between 7 & 10 degree's

any input on those 3 would be appreciated.

Some other questions.
Iv seen diffuser that are the full width of the rear of the car. Others that are only focused in the middle of the rear.