There are a few threads with this very topic here. If you don't want to go the fan route, make the hole bigger and install two 1/2 inch copper pipes. The pipes will extend into the fridge a couple of inches and go up the tower to just below the shanks. Then fill the tower around the copper with expanding foam. The copper will pull the cold all the way up the tower. Your beer lines will travel inside the copper to the shanks keeping the beer lines cold. Cheap and simple.