We ordered groceries online because of COVID and it came with freeze-blocks that were water bags. I tried heating them on the radiator at home and putting them on the dash in the car.
It kept the fog off the inside of the window just long enough for the engine to have gotten warm before I needed do use the heater. It's perfect in the Prius because if you use the heater before the engine is warm it keeps the engine on all the time.

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2016: 128.75L for 1875.00km => 6.87L/100km (34.3MPG US)
2017: 209.14L for 4244.00km => 4.93L/100km (47.7MPG US)