A quick-fix for inadequate insulation follows:
Get a roll of “Reflectix” (called something else at Lowe’s & Home Depot, but the same thing). This is the foil-faced bubble pack insulation.
Plan & cut the minimum number of pieces needed to cover the interior of your box. You will have to (temporarily) unfasten your evaporators, cold plates, or whatever in order to effect the installation. Don’t disconnect the refrigeration lines!
Carefully cut the pieces so that they fit tightly against the adjacent wall with a “press fit”. Use one piece for the front, bottom & “back” (hull side) of the box.
“Cut out” the piece that will be “behind” whatever refrigeration equipment you unfastened and seal those edges. That way you can easily remove “the rest of” that wall (or top) insulation.
Wrap all edges with foil tape. If you want, you can get even more insulation value from doubling the thickness of the insulation. If you do this, I’d suggest NOT joining the layers with foil tape around the edges, but instead, tape the edges of each layer and put one on top of the other.
Don’t worry about cutting out for the drain hole in the bottom.
Depending on your installation (this won’t work if your evaporator is attached to the horizontal top of your box), the following step is optional, but very effective…
Cut a single piece of the insulation that will “float” on top of all your goodies above the level of the evaporator or holding plate. If the box is less-full than this level, you will have effectively reduced the volume of air space which it is necessary to keep cold. If it’s more-full, then it will “self-adjust” as necessary. It also makes it less-important to quickly close the main lid of the box when retrieving items from within. If you have a “spill-over” between the freezer box and the refrigerator box, this “floating lid” must remain higher than that spillover (fan/hole/vent)
You’ll possibly have to adjust your thermostat higher in order to prevent freezing the things that you were formerly merely refrigerating. (or decrease the volume of spillover air into the reefer box from the freezer box).
If you have a spill, or are turning off the box, just pull out the insulation (except for the small pieces which you’ve trapped behind the re-fastened refrigeration equipment) and rinse them off and let them dry. Clean the rest of the box as usual. If you go off and leave the stuff in the box (dirty) with the box turned off, you could get a mold situation going there.
I put this in years ago as a stop-gap in my non-refrigerated ice box & freezer box in order to make the bagged ice work longer… I’m still astounded at how LONG that ice holds up, and the “need” for refrigeration for my weekend-to-five-day excursions has basically been eliminated (I’m still using just ice or ice & dry ice).
The weight of the cans, bottles, ice etc hasn’t “popped” any noticeable percentage of the bubbles. The items in the box don’t make near as much noise rolling around, and there’s less of a chance of breaking any glass bottles that may be in there (if you keep the gin cold, the ice in the glass lasts longer).
The stuff’s not very expensive, and it would be a good way of determining whether merely augmenting your box insulation would be enough to permit you to operate your (existing) refrigeration on your solar panels.
My favorite outdoor bar (now gone) in Rockport, Texas was unable to keep the margaritas in the machine frozen due to the warm air that blew over the clear plastic “bucket” (on top of the machine) that contained the mix. I suggested this to them, and it worked like a champ. They just wrapped the machine in the foil faced bubble pack…voila…frozen margaritas.
You can do the same for your “igloo” type cooler, particularly if it’s used on deck.
I made up hatch covers for the interior of my main hatches out of the stuff. Makes a huge difference. If they get ratty looking, just toss them and make up some more for a few bucks.
There’s no reason you couldn’t “quilt” this into exterior canvas hatch covers/companionway covers either.