I have recently bought a BougeRV CRPRO 20 L compressor fridge/freezer, capable of going down to -20 degrees if needed. The interesting point with this is that it has a compartment to take a battery which can be bought from them as a package so that it becomes a self contained unit. The fridge draws no more power than a peltier cool box, has normal or eco modes so only draws 36W in eco and 45W in normal, but unlike a peltier coolbox it is not continuous. It has built in battery protection with 3 preset thresholds. The battery which fits into a compartment in the top has an accessory socket output, a USB A output and a USB C dual direction port, and a built in torch. As well as the USB C port for charging the battery there is a DC power connector with cable supplied to charge from a car accessory socket, although this might need extending if your socket is at the front of the car. The battery supports simultaneous charge and discharge with a readouts for both. So it will power the fridge and charge when the car is powered up. The battery has a capacity of 277Wh or 15Ah. Mine came from BougeRV themselves, UK store, but Amazon also have them, so there might be a better deal at one or the other at the moment.
The only drawbacks are price compared to a coolbox if bought as a package with battery, and in my MG4 when it is installed in the boot it tended to block out radio (dab/fm) reception. I have found however that if its covered with several aluminium lined normal cool bags this screens it enough to make reception almost normal except in the weakest areas.
