Don't rig it to the fridge light. When you shut the door the fan will go off.
I suggest getting a 4w, 115vac fan (see http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180208066702) and just rig it to the main power bus...no need to have a separate adaptor and outlet. If you also put a...
But how much does the fan cost? I can't get a 4w fan for less than $7, let alone the project box, etc. Copper tubing is MUCH cheaper than that.
Have you considered that having two open tubes go into the tower creates a convection effect?