I have to stronhly agree with Steve D about the Isotherm. Actually I recommended it to him. I purchased a unit from them for my '81 H33 (I used to be a "Cherub"!) and installed it myself in a less than a day. The only time consuming part of the installation was building a shelf in one of my underseat lockers in the salon to mount the compressor on. The reason I installed it under the settee was simple. 1) it was immediately forward of the reefer box (maybe 2-3 feet) so the refrigerant tubing runs were short, 2) close to the galley sink thru-hull fitting which was replaced by the Isotherm unit (the cooling 'fins' are integral to their propriatary thru-hull), and 3) I didn't need to 'vent' the compressor anywhere or even install and air grills.
That compressor (Danfoss 50??) DOES NOT NEED AIR CIRCULATION!! You can keep it in an unventilated closed!. And it's so quiet that the only time I realized it was on was by checking my Xantex amp draw monitor. We're talking SERIOUSLY QUIET!
In terms of efficiency, I installed an 80W solar panel to feed the house bank while the boat sat on her mooring during the week. For the record I had a 400 aH house bank (a pair of L16s). I installed the refrigeration unit in May, started it up, and shut it down in September - almost close to 4 months continuous use. The only time I ever got to the boat and the house back was depleted almost down to the 50% level was after a week of continuous overcast and rainy. Otherwise the 80 watt panel fed her all summer without issue.
Excellent unit - especially for a DIY project....