My understanding is as follows:
The reason that a warm air intake works on a gasoline ICE (with fuel injection) is because you actually reduce the density of the air coming in at a given throttle position. The ECU attempts to maintain a given A/F ratio and reduces fueling accordingly. But now you are making less power at a given throttle %. So you dip into the throttle a bit further to get back to the power output you need, and thus match the oxygen intake, fuel consumption, and power level you would have had with a cold air intake at the original throttle %. Only now with the throttle open further in the case of the warm air intake, your pumping losses are reduced because theres less of a stupid restriction in the way. Hence another advantage of the diesel engine (no throttle to arbitrarily reduce engine efficiency).
Increasing the intake air temperature (IAT) has an effect on the quality of combustion. The exact effect is debatable, and multiple papers have been published to investigate the nature of this parameter upon combustion efficiency. My opinion has found that warmer IATs result in less power from my diesel engine. One can logically conclude that this is due to poor(er) combustion. Which means your fuel is not being used to its fullest. So without the benefit of reduced pumping losses, theres no reason to raise the IATs on a diesel.
Your rough grill block sketch looks good to me!
I had the same problem with an upper grill block (engine intake located here) so I solved mine this way:
I'd like to re-route the intake someday and close off the upper grill for good though.