Yeah i have an old ice box that i converted into a fridge freezer.
could be better and long term i will rebuild
it all as i want a larger freezer and a larger fridge, for my remote
But, in saying that what i have works perfectly well if you keep it filled with something.
is a little small for the box that i put it in (around 80 liters) but with a small fan to help it circulate the air it works well and only uses a tiny amount of amps
It started out as an ice box with a shelf in it. I then got a portable fridge like your waeco / engel types, a 40 liter one to be exact with a danfoss bd35 compressor in it.
"WITH GREAT CARE AND PATIENCE", i removed all the plastic crap leaving just the compressor and the evaporator plate from this all in one piece.
then "WITH MORE GREAT CARE AND PATIENCE", i carefully straightened the plate, cut a slot in the side of my ice box and poked it in and then curved it to shape in situ inside the box.
So it was still in one piece and still worked, just needed to seal the slot, wire the extra fan of the compressor fan and give the beast some power.
To seal from the shelf to the top of the plate i just cut some perspex to shape and it acts as a cover over the freezer inside the fridge from the plate to the shelf.
So you can see through it, store fridge items on top of it, and it allows the freezer temp to be maintained better.
All in all, about three hours work, the portable fridge you can pick them up secondhand for $200-300, the little fan came out of a computer power supply.
If you need to insulate the box with out rebuilding it just to get going just glue some polystyrene in side.
It will make the box volume smaller, give you more freeze capacity from a smaller compressor, or you could paint
inside the fridge with a ceramic vacuum sphere insulating additive, mixed into some paint
, two of three layers will help a lot.
lastly make sure that the plate sits of the wall of the fridge by about 5-10 mm so that you can use the coldness from both sides of the plate.
Best of luck,