Always wanted / needed additional freezer space for longer roadtrips so this is how I solved the problem. After careful measurements, I found that the Alpicool C40 Portable Refrigerator 12 Volt Car Freezer 42 Quart(40 Liter) would be the best fit. Can be used with 120 or 12 volts.
I made a slide out tray with 100 lb.Full Extension slides and installed a 12 volt receptacle in the compartment.
I have used the Freezer/ Refrigerator on two trips now. One 4 day trip as a refrigerator and a 35 day trip as a freezer. It has worked flawlessly. The compartment was previously used for drink storage or misc. ****. I even made the accessory pouch that carries the 120 volt cord.