Framed with 2x4s and the sides are 5/8" plywood. The bottom is MDF since I had it already. If I do it again or make any changes I would use some of the whiteboard material you can buy at HD instead. More slick and easier to clean up. Once I got the box built I caulked all the interior joints.
The insulation is 3/4" and I doubled up on the ceiling, back and side walls. Used 3M adhesive spray. Didn't hold so well in some places so I used a couple roofing nails. After I installed the first layer of insulation I duct taped all the interior joints then cut the second layer to overlap the first joints. I duct taped these also for more protection. After I cut the door out of the front I glued the insulation to the front. I then cut the insulation out for the door smaller so I have overlapping edges to reduce air flow more.
To hold the box to the fridge I first butted it up against the box then used a strap to hold it in place while I attached the trim all the way around. I had some quarter round laying around so I cut that for the front and top, attached it to the top of the fridge and side of the box using liquid nails. The back didn't have to be pretty so I just used a short length of 1"x2" screwed to the box and glued to the fridge. I left it to dry overnight and the next day it was stuck. Just to be safe I used duct tape on the inside too, even though I'll never move it.
I picked up the temp controller and wired it in according to the directions. I've got a dremel w/ a small cutting bit and I cut a hole in the edge of the fridge to mount it. I ran the temp sensor around the back and drilled a small hole to get into the box. I have it at the opposite end of the chamber. You can see the black lead laying against the back wall.
One thing I didn't get a picture of yet is the computer fan I have wired in sitting by the freezer compartment of the fridge. I debated wiring it in with the love controller so it only switched on with the compressor but I think leaving the fan running keeps the compressor from turning on so frequently. Same principal as a variable speed furnace fan. I'm using a 5v adapter from a cell phone or something. I first used a 12v and that was way overkill, all I'm trying to do is keep the air circulating.
Lastly I do have condensation issues but Damp-Rid helped fix that. Hope this helps.
Oh and BTW I think this has improved the quality of my beer more than anything else. Summer in GA made it hard to keep the temps below 75 if I didn't stay on top of it. Now its smooth sailing, all I do to cold crash is adjust the temp