You can use a copper pipe and insulate the areas outside of the tower and fridge. Inside the fridge put a Tee at the end of the copper pipe and run the beer lines out the bottom. Then on the leg part of the Tee attach a vinyl hose to it and at the other end connect it to a fan source. If the fan is to powerful, your fridge is going to cycle like crazy since the fan will be sucking all the cold air out of it. Get a variable speed fan so you can dial it down to just enough where it's keeping the beer lines and the tower cold.
This is what I did on the cheap with a trip to home depot and an old PC fan: http://www.homebrewtalk.com/f51/kege...lation-412148/
Not only does it keep the tower nice and cold but it also circulates air around the kegs in the fridge. The all weather electrical box I used has two ports on it. One port has the vinyl tube going to the copper pipe and the other is just opened into the fridge.
EDIT: To seal up the hole I made where the wires comes in for the fan, I used Plumbers putty (it's like clay). It doesn't harden, creates a nice air / water tight seal and in the even that I have to change the wire or want to add something else through the hole, it can come right out.