maybe you find this
a worthwhile read.
On a side note, maybe you want to mind about an alternative fixing method for your strakes (vertical fins) in the diffuser.
If space permits it ( and I don't know, because can't see the top of the diffuser), you could mind about using
slots in the diffuser top/roof and fix the strakes on top with a lateral bolt or rivet, that would clean up the
air flow quite a bit, and you may see less flow separation where you don't want it.