I am actually in the process of building my own AC unit cooled kegerator/keezer/fermentation chamber. I have found that if you can over-ride the temperature control on the AC unit (electrically with modern ones or bypassing the old mechanical temperature regulator), then you can get them to cool down below freezing.
You mentioned that you had an idea for the temperature control so I won't go into that. As far as air flow goes, it is a fully enclosed system. The cooling side of the AC unit pulls air in across the radiator and blows it out through a duct just above it so you don't need to deal with venting or drawing in air. All you need to do is mount it like you would in your house, but instead of sticking in a window into a room, you are sticking it into a small box filled with beer in various stages of progress.
On the construction side, there are a number of ways to go about it. The simplest is to use foam insulation panels (white, pink, or blue) to build the box and provide insulation. The foam isn't really strong enough to allow for multiple levels, but for a single level it should work. Also, if it isn't providing enough insulation, you can always double layer it.
I am finishing up the temperature controller for mine now and should have a blog post up on it in the next couple weeks, but to give you an idea,
Here is a link to my teaser post.
Fermentation Chamber built with window A/C unit
Linear Mode
