In order to power my fridge and other accessories while travelling, I installed a dual battery system in my Defender using a Blue Sea Systems Automatic Charging Relay.
This system is rated to 500 amps continuous. This is ample when combining batteries to jump start the vehicle from the secondary battery should the primary go flat. This unit also comes with a dash mounted switch allowing easy connection or disconnect of the 2 batteries. Technical specifications here.
I used a Full River 120a/h AGM deep cycle battery as my secondary, which is probably the largest size you could fit in a Defender battery box.
I manufactured a dual battery tray for my Land Rover out of 3mm plate steel and 25mm RHS with two mounting tabs underneath.
I completed the installation with an Anderson plug which will allow me to connect accessories such as my Inverter without having to remove the seat and battery cover.
The completed installation. A snug fit.
If the video did not display above you can watch it here: http://youtu.be/YPM-leVEfMc