Crash,
I put a small ( 6'x6' ) walk in fridge in my auxiliary kitchen. Bought the door off Craig's List....door is from a 'regular' cooler, aluminum skin with foam inside, handle, etc, and came already framed in the same material the old cooler was built with, which was 4" foam with aluminum skin on each side.
I framed in the room in one corner of the kitchen, then bought some 2" foam board from a local roofing supply place, and used those long ( 8"x 1/4" ) screw/spikes w/hex head they sell at Lowes for log cabins to hold two layers of foam to the walls, ceiling.
Floor is tile on concrete. I did not insulate the floor ( which is recommended ) because my planned used of the walk-in will be pretty limited. I plan to use it just to hang meat ( pork, beef, deer ) killed here on the farm for the length of time it takes to process them in the kitchen ( which is really a meat cutting and canning room ).
For refrigeration, I plan to use a 10,000 BTU window AC unit and one of those Coolbot (
Click HERE for info ) temp controllers. The Coolbot site also has wealth of info on building your own cooler.
Right now, I'm remodeling the house kitchen, and "the man cave" ( as my wife has named it ) is being used for our only kitchen, so I haven't had time to finish up the cooler and test with the AC, but that is on tap for this fall.