Paul,   how does one relate the size (surface area)  of a radiator to
the size of louvers required.  Do you still like the idea of a few
larger adjustable louvers (more like cowl flaps in size and shape)?
I am  pondering how to build  louvers that are shaped to match the
cowls shape.    Any suggestions would be appreciated.      Jerry

The exit area when in cruise should be a bit smaller than the intake
area. In climb out the sky is the limit as aerodynamic  drag has absolutely nothing
to do with climb. As few as three will work but I suspect four or five
might be better.