I wasn't as concerned with the conducted heat coming off of the inner vertical wall of the rear reflector box and into the engine compartment as much as I was concerned with allowing the hot air flowing through the rear box itself to escape rather than stay in the box to cook the paint off the rear panel and also burn my rear F/G bumper. The conducted heat from the box would raise the temperature of the engine compartment air from a physics POV, but that proved to be minimal as the oil temp never reached over 200 deg in this configuration (at idle in traffic or on the race track) and the paint no longer cooked/bubbled off the rear panel. The F/G bumper didn't burn either after that.
Although I didn't measure the fan inlet air with a thermocouple before and after the changes, the real-world results I experienced for my particular situation satisfied me. I can't do any further testing on the car b/c it sold 3 years ago. I thought I'd offer the OP an additional option to do something with the unsightly, reflector-less rear panel. I'm obviously not a physicist.
