Low temperature causes higher fuel consumption, so I tried an old trick.
In a car accessory shop, I've found and old Fiat 127 radiator rubber cover. The owner was pleased to sell it to me at a discounted price, since he said nobody uses it anymore.
Colour and size are just fine for the Yaris.
I just had to cut some holes to fit 3 plastic cable ties to grab it to the rear mask.
There are even some windows that can be opened to regulate air flow in warmer days.
Results are encouraging.