Given the consistent results I've got with my previous modification, I've dared a little more.
I fitted some cheap pipe insulators in the remaining slots of the radiator grill:
The lower insulator is kept in place by pressure from the license plate, while I fixed the upper one with a couple plastic ties.
I was worried this type of insulation will prevent the engine from refreshing and will cause it to overheat.
As a stress test, I drove for an hour on various medium/high speed roads and some steep hills.
Even though the weather was sunny and the ambient temperature was 7 degress, coolant temperature reached 90 degrees and didn't go any further.