I'm late to the party again but this may help someone.
Last summer I picked up an enclosed trailer (5 X 10) that was used and pretty well beat up. It was a Haulmark brand however and that means it's quality. I took it down to AZ and worked all winter on it to make it into a Cooler Trailer with a 12,000 BTU AC and Coolbot.
I installed 4" of foam on the walls, ceiling and floor as well as the swinging door. I lined it with 1/4 inch plywood on the walls and ceiling, 3/8 on the floor. I cut a big hole and installed the A/C unit in the front end and installed a standard electric box with a pass through and outdoor marine plug in. I then caulked all the seams and cracks and used a whole gallon of Kilz paint on the inside and then a whole gallon of porch and floor paint over that. (I also fixed the hole in the roof from a tree branch, installed new tires, repacked the bearings and sanded and repainted the trailer frame.)
Lastly, when all that was done I installed the CoolBot unit (easy) and had a local artist put our farm logo on the exterior.
Then we pulled it back to Wisconsin and set it up next to our farm stand. I bought wire metal shelves and screwed them to the inside.
I had a bunch of photos but lost them when my computer bit the dust. Here are a few I salvaged.
A couple of notes: Using 2" rigid foam ($20 per 4x8) was the cheapest way to insulate. I used cans of spray foam to fill cracks and seams. I found the niftiest thing to make cutting foam easy, a Hot Knife from Harbor Freight. Finding 4" and 5" screws to install the foam was a pain. And expensive. I added big tie down eyes in the four corners so I could use the trailer for other things too if necessary. If one wanted to spend more $$ and less time I would recommend having someone spray foam the interior with 4" of foam. Or buy one of those kits and do it yourself. Getting the foam on the ceiling was the most difficult part of the entire project.
So, how does it work??? FANTASTIC.
I just put 25 heads of cabbage inside and it is 42*, right where I set it. I cannot say how nice this will be to finally have refrigerator space! We decided on a trailer rather than a walk-in because we can take it to the Farmer's Market, and even to the field when we harvest.
All in All I have about 1900$ in the build with $500 being the trailer purchise and $200 for new tires.