Really admire your persistence in dealing with this problem!
I think I'll try to fashion a similar deflector on top of the fins, as I can see where heat would get trapped in the space above.
Also, with mine, at the top of the wooden baffle (piece of panelling) there's a space of about an inch and a half (horizontally) between the baffle and the fins. I'm wondering if much of the airflow is taking the path of least resistance and just exiting the outer vent without even entering the cooling fins.
I may take a piece of aluminum and extend the baffle up a bit further and bend it inward toward the cooling fins.
2008 GMC 2500HD, 6.0 gas, ext cab, 4WD, 3.73
2014 Rockwood Signature 8329SS
Equal-i-zer WDH (12k)