Any of the Aussie-built units can freeze. Look at Engel, for examples. Expedition travel websites have tons of reviews and all the info about the latest models available from importers here in the US.

When searching for a small freezer don’t forget that it is very easy to add a 12volt to 120 AC inverter to your car.  That way you don’t have to look only for a 12 volt freezer.  Any freezer will do if the size is appropriate for your needs.  Just make sure that the inverter will handle the required power load.  I run a large freezer in the bed of my pickup truck all the time using this method. 

Make sure you look for a unit with a true refrigeration system.  Most of the ’cooling’ ice chests use a device that will cool about 40 degrees from the ambient temperature so if it is 100 outside of the ice chest, it will only be 60 inside.

I love my Dometic. I  have a CF-25 –  but I think all their CF and CFX models are basically the same in principle, they just have different case size shapes.  It’s a 12V  DC, super sturdy, high-end compressor style unit (”true refrigeration” as opposed to one of those thermoelectric coolers that rlbrooks  was criticizing above, accurately). It’s got digital controls  that allow you to set the temperature  with real precision-whether you’re refrigerating or freezing.  It’s Silent and uses almost no electricity. I  would not recommend inverting your vehicle’s 12v line and plugging any old freezer into it. I’m surprised that works at all- that truck  mentioned above must  have a large engine that’s running a lot. It costs you 5 watts just to invert your volts .  You can run my Dometic off a small  Solar panel, without an inverter. 

http://www.dometic.com/USA/MS-11346-Marine/PG-11355-Refrigeration/PG-11382-Portable-Coolers-Freezers

 Oh, and it fits  perfectly on the floor between the front and back seat, or the front seat passenger side .

The Norcold portable refrigerator/freezer (now apparently owned by Thetford) will cool down to 0 F. I have one of these dating from the 1990’s that we used for camping. Still works fine. The current design looks like it has a plastic exterior–mine is all metal. Works on 12-24 VDC or AC.

http://www.thetford.com/products/refrigerators/acdc-portable-refrigerators/