What is the drag penalty for flush fit window louvers ?
 

By " flush fit", I mean louvers that do not rise up past the curve of the body.
When seen from the side, they are invisible.
Something like this : https://ecomodder.com/forum/attachme...1&d=1673708673

Well, of coarse it varies by design and other variables, but is there a graph that shows a certain case study for flush louvers ?

People with those louvers generally don't care about drag in a meaningful way for fuel mileage. Might be searchable data, but I suspect it's rare.

I'm adding louvers ( instead of a window ) to my car because Plexiglass is so expensive. When I started seeing all of these Plexiglass shields being installed at my job, I thought I had hit a gold mine.
I was sure they were soon going into the dumpster, and that I could find plenty.
Here we are years later, and the Plexi is still up, but now has graffiti ( names scratched in ) on it, or is cracked and yellow.

case study
 

I've no knowledge of such a study.
If they were 'airtight' they could perform a drag reduction function, as with the ganged, backwards-facing steps illustrated by Fredrick Lanchester in his 1907 book.