A simpler fix as opposed to installing a removable damper baffle plate to
fit inside the NACA duct.  Install two nutplates on a baffle plate and use
screws through the cowling skin to attach the baffle plate.  You can adjust
the temp flow by using different plates of various  size and configuration.
This is a simple clean neat and light weight fix to solve the problem.

Bob Smiley
N94RJ