Hi Ola. I fitted a complete refrigeration unit into the coolbox of my 275 last year. There was plenty of room for the unit and motor on the shelf underneath the sink on the port side of the companionway, although it was fiddle getting acess to the final wiring and connection to the cold-plate . I'm not sure whether a 27 has room for a coolboz in the same place as a 275. The insulation was greatly improved by squirting lots of builders foam round the box - cheap and easy to do. I leave the fridge on whenever the engine is running, and it is not too power greedy Good luck!
Tim Howard