anything more than 2 corny's is going to be risking going beyond what the fridge can cool. you can build it bigger, but it only has so much coolant and so big of a heat exchanger plate to cool with. like i said, keeping things at a temp is easy, it's bringing them down to temp that hurts.
for the amount of time and energy you're going to spend expanding a mini fridge to a size it can't cool, get on craigslist and find a cheap/free fridge and put a temp controller in it. my regular sized upright fridge holds 4 carboys or like 8 corny's without a top shelf that could probably double the capacity.