I have a 13cu and it provides quite a bit of room, you should be happy with 15.7. Have you searched the
DIY section yet?
There are some great projects. Most just go with a simple collar for keezers but it sounds like you might want to take some of the kegerator conversion threads to heart if you want to go with a tower. I personally wouldn't want to do anything but a simple collar as I'm going to keep the freezer dual purpose (serving and fermentation on and off).
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