I converted an existing cool box on Kate to a proper fridge using an Isotherm Compact 80 kit. It was dead easy and it is very efficient, partly because the cooling compartment is very much smaller than the capacity of the unit (30 litres for a max of 80) and it is very well insulated. Remember, you only have to pump out the amount of heat that can get in.
The cool beer is wonderful, so is the fresh milk and if I catch a big fish I can save a fillet for tomorrow.