[gumby79]

The 0.125x3" flat wing accrost the hinges on my cap was welded with ~2.5-5°angle of attack to :
Create a small amount of converging duct.(1/8" closure over only3" isnt going to do much if anything.)
Be above the turbulance caused by the cab cap gap.
Cause a slight down wash promoting attachment sooner.
my tuft test video showing attached flow Downstream

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Julian this looks to be a proper place for you to apply the eroding clay technique.

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Here's a picture of the inverse of eroding clay. Additive aerodynamic sculpting.


Pay close attention to the upper screw holes in a license plate that allowed are passed through the bumper due to the pressure differential the scallops and the shape of the entries.