I assume you plan on setting this AC unit on top of the refrigerator with the back of the AC unit essentially flush with the side wall?
A standard window unit will not work well like this, if at all. As someone mentioned earlier, window AC units pull air in around the sides and blow it out the back. They need clear open access around the sides to pull in cool air and blow it through the coils.
Even if you leave air space around the sides of the unit, it still won't work well/at all because it is pulling the hot air it just exhausted through the coils right back in. In open air this still happens, but to a drastically smaller degree. When you enclose it the hot air is ducted right back into the unit.
What you are proposing would be best suited to a "sleeve unit" type of air conditioner. Unfortunately you won't be able to buy one of these for $79 at Walmart.
