Thermoelectric cooling is not very effective because it is limited to about 40F below ambient. On a very hot day (say 90F) you fridge is only going to be 50F. That is too hot for very effective refrigeration.
On my last boat I used 4" foam all round a small (say 2 cubic foot) icebox. 40 lbs of ice would last a week in Mexico.
On my current boat I am using mostly 6" foam with a small 2.7 cu ft. old fridge liner + an "icebox conversion unit" from NovaKool. I have to keep it on the highest temperature setting or it will freeze the contents.
