I was lucky enough to have a friend who worked for the railroad and was tasked with replacing several of those big silver metal sheds they have by railroad crossings and he was kind enough to give me one of the old ones thats about 6' x 6' and 8' tall. I cut a hole in the side for the window unit and hooked it up to the cooler bot. For insulation I cut some rigid foam insulation into 5-1/2" strips and installed it inside the container in the same manner that you would frame it with 2 x 6 studs, except these were made out of foam (used foam instead of lumber because that avoids any thermal bridging from the lumber and since the metal enclosure was self supporting all I was trying to do is create a cavity between the metal shell and the inner space), then I installed rigid foam insulation panels as the interior wall. once the rigid panels were in I poked some holes in it between each set of the foam studs and shot a bunch of expanding Polyurethane foam insulation into the cavity. Next time I go out there ill try to remember to take some photos. It works really well. The cooler bot barely needs to run since it is outdoors and I primarily use in the winter so really it only kicks on for a couple minutes here and there on warm days. I actually ended up putting a small space heater inside it with a cheap inline plug in temp controller from amazon to keep it from turning into a freezer during long cold spells. I keep the heater controller on 38 degrees and the cooler bot on 44 and there is enough temperature difference between the two settings that I don't have issues where the heater and cooler both end up running at the same time.
I got a walk in cooler for free with a broken refrigerator unit. I bought a device called a coolbot and wired it to a home window a/c unit. $250 later i have a full 10x10 walk in cooler. If you can get you hands on a free/ broken one look into that.
