any gaps you see were filled in with liquid foam.
After we added the layers of foam inside the frame, we added a full 1.5" layer of foam on the inside and back, cutting only a hole for the AC unit.
We added the top (unfortunately i don't have pictures) overlapping the same way after we installed the foam and screwed it into the back and the sides. this made everything so solid, i could stand on it confidently.
Here is the cabinet as it is now:
We installed hinges on the door and lined around the doors with stripping. You can see the extra layers of foam here. We also put a thin piece of plywood on top of the bottom foam so it wouldn't get damaged from moving carboys in and out:
And here it is full of beer. it fits six carboys, easy. you can see the ac unit in the back and we drilled an additional whole for the cable to come out and attach to the external temperature controller.:
The foam pieces are all glued only to their individual panels, so it can be taken apart piece by piece (top, sides, and then back) and moved. We added liquid foam to seal them a bit, but nothing a razor blade won't take care of.
In addition, here is some math my friend did for the energy it would take to cool in regards to space, insulation, etc. it works pretty efficiently. it kicks on every few hours for a couple of minutes and maintains a nice ambient 58°F:
