I own a h280 which only has a cold box ie. no compressor but insulated walls. i purchased a Isotherm by Indel Marine, they supply a Danfoss Compressor and evaporator that is pre gassed and connected by two quick connect couplers. The whole installation is very simple all you have to do is mount the evaporator inside the box with four selftapper screws, drill a 30mm hole through the side wall push the copper tubing with quick connects through and re connect to compressor (you dont lose any gas doing this) mount the compressor in an convieant place (under starboard bunk) add power and the thermostat that comes with the package and switch on. make sure you size the evap plate and compressor correctly for the size cooler box. the evap plate can be bent to suit box size. i bent mine so three sides have cooling (rather to large than to small)
Doing it this way alliviates the problem of adding an addittional cooler box