We have two refer units on our boat. An original FrigoBoat--1986 era and a mid-90's era Sea Frost for our freezer
(on a 1986 Beneteau
First 42). The person we bought the boat from drilled holes in the top edges of the boxes and injected foam insulation
around both so they are well insulated. Inside the factory lids, there are second clear plastic lids that we set 1-1/2" closed cell foam cover pads on--the pads having been wrapped in pieces of silver mylar film cut from survival "blankets" sold by West Marine
. Both the refer and the freezer
will freeze bottled water--or lettace--rock solid over night with the thermostats turned down too much. We usualy keep the freezer set at 7 (out of a possible 10) and the refer set between 4 and 5. The freezer easily keeps a gallon tub of ice-cream, or a box of ice-cream bars or popcicles, solid (which makes us the most popular boat in an anchorage when my daughter invites the kids
over for Ice cream cones!).
Generally we put bottles of water
and GatorAid in the Freezer, next to the plates, until they're nearly frozen and then transfer them to the refer box for the day. We also have 4 aluminium closed trays with plastic divider inserts that we fill will water
and stand in the freezer over night, and then transfer the "cubes" to a zip-lock bag the next morning for drinks and such during the day.
With the insulation
we have--and if I can keep the girls out of the freezer too often--we use about 100 amp hours to run both units for 24 hours here in Florida
during the summer. Less if we're someplace where we can plug
into shore power
to run the AC on the boat as the units are both air-cooled.
I think the key to these things is the insulation and particularly at the tops, which I think are frequently overlooked.