We spent six years cruising in the Sea of Cortez
and as far south as El Salvador -- summer temps were often over 100 for days on end. Add to that our refrigerator
was next to the engine
and next to the companionway
with the sun beating down on the (dark) teak
surround. We did everything we could to add extra insulation. Several ideas:
We got a rubber mat, cut it to the exact dimensions of the counter top (less 1/2" all around) and had a canvas
shop put a vinyl cover over it (the seams made up that 1/2" so it was now the exact size of the counter top) and laid it on the counter over the fridge. Since this was my main food
prep counter, the vinyl made it easy to keep clean (I thought about the neoprene idea, but knew I couldn't keep it clean enough without some sort of cover). To get into the fridge, I just flipped the cover back.
We got two auto windshield reflectors -- duct-taped one around the outside of the refrigerator
with the silver side out to keep that teak
enclosure from heating
up, and laid one inside the fridge on top of all the food
. We eventually cut that one in half and just used it over the side with the chill plate, which we got into much less frequently (there were two openings side by side, but we got into one side much more often than the other, and having an extra layer of insulation there just made it too hard to get to the drinks). NOTE: While it wasn't exactly pretty, the reflector around the outside of the compartment did a lot to reduce our energy consumption!
Friends on a boat coming down from the US bought some of that blue insulation foam (hard sheets) at Home Depot and cut pieces to fit in the top of their refrigerator and inside the lid, then used silicone to hold them in place. They said it did a lot of good, but we couldn't find that type of material where we were, so I can't personally vouch for it.
We did get some of that Space Age insulation and used it inside the engine
compartment on the common wall with the refrigerator. It didn't do as much to help the refrigerator as we had expected, but it did help to some extent.
Another thing that almost every boat in the Sea of Cortez
did in the summer was to hang something -- an auto windshield reflector, a piece of white Sunbrella, whatever -- on the outside of the hull
over the refrigerator. Dark-hulled boats said this made a HUGE difference to them; we noticed a small improvement (we had a very high gloss new white paint
Other things that will help are loading up the refrigerator with drinks after sunset so that the compressor
is working to chill them down with the cooler night air and developing a strict organization system so that everyone on board knows exactly where things are and the refrigerator doesn't have to stay open any longer than necessary -- and putting the drinks in the most accessible place!