Okay so I just measured and I can't fit a 120mm fan. Would this product work? http://www.sharkacomputers.com/fanad8012fan.html of course I would want to mount it on the inside of my case.
You can use an old scratched CD to use as a template for the hole. I learned that from someone else on this forum.
http://www.sharkacomputers.com/fanad8012fan.html
Your link was messed up. That would work okay. But you might have an issue mounting it on the inside because of the size difference.
What you could do is make a triangler prism out of aluminum sheetmetal.
Give it just enough angle so you can mount the 120 fan on the longest face. Mount the box to the back of the case with sheetmetal screws or pop rivets.