Our 39L Engel has a home made cover, canvas lined with woolen blanket. (Lid has double layer of blanket under the canvas) Use it as a freezer on our 3-4 week trips with out any problem, haven't checked the actual temperature for decades but everything remains solid. Also have an Engel combo (60/80L?) we transfer frozen to fridge section daily to thaw for evening meal. As the freezer section empties we transfer frozen from 39L. Usually the 39L empties in 3rd week and is shut down.
Both these fridges are on the back of the ute and a 110AH AGM supplies the power, charged with 200W solar and alternator while driving. If by chance cloud rolls in battery is topped with generator powered battery charger.
Meanwhile the 3Way in the van is running faultlessly on gas.