I'll never do these, but props for nice write-up and pics.

Just a heads up for folks if you are mounting these up on composite fenders space the template away from the edge a bit to compensate for the thickness of the composite. It will save you a ton of time.
https://cimg3.ibsrv.net/gimg/www.mia...213bab1fe.jpeg

Ok thank you, more pics of whole car please. :D

EDIT: Watch or re-watch Ryan's install video before tackling this and don't be like me My lower flange ended up short of the template (why I had to use painters tape to mark a new cut line) because I was flexing the fins themselves to help the lower flange meet up with the fender. Seems that's making the vent smaller than intended and the fins have a bit of a bow in them, woops I'll go back and try to fix it later, but uh, don't do like that.

I prolly shouldn't have done hood and fender vents all in one day, but here we are. I'm pretty haphazard and put zero focus on attention to detail anyways (despite the Coast Guards best effort many moons ago), but all done for now. I might go back later and install them from underneath as is intended, but also it's a racecar and I kinda don't care. I haven't been on track since September after roasting a motor and still have a tooonnnn of projects to churn through so the details will have to get cleaned up later...maybe...probably not.

Used blue tape to mark 20" from rear corner of fender
Applied template with help of one of those card thingy's you use for laying down vinyl (my brain is fried right now)
Made sure template holes lined up with the actual vent then drilled
Used my wood work bench to help bend vents into shape, whichi prolly took longer than anything else and I still didn't bother making it perfect
Dropped rivets into the two template holes drilled previously (just looseley hold vent in place) and then used blue tape to mark off the outer edge of the flange while applying a little pressure to it so it would locate to approximate location when tightened down. Then used more blue tape offset about the width of the flange to mark my cut line.
Cut out the fender like a heathen with said angle grinder. This is where I really don't care, roasting paint and doing my best not to be a total hooligan, but mostly just getting the job done.
Finally, riveted in the vent/block off plate with the two template holes drilled previously and started working my way around.
Done.

I did this the least fancy way possible.

Good tips from everyone, glad to see more of these getting on cars!

Yep, fins should stay straight (do not bend a curve in the fins). That would indeed narrow the overall footprint of the louvers as you found.

https://cimg2.ibsrv.net/gimg/www.mia...295658cf5.jpeg
After mounting mine I took it out in the rain on slicks just to play.
Quickly discovered the amount of sand and rocks being thrown into my oil cooler and headlight area past my cut fender liners was unacceptable.
Have a large oil cooler mounted in front of wheel well on other side of vented liner.
Came up with this to block dirt from headlight area. Just plastic heated and bent to shape after lots of fitting with cardboard template.