If there's an rear 120mm vent, Koolance used to do a quick disconnectable mount that u can mount on the back to hang a Rad over. I had my EX-480 Rad on the back of a small Nvidia branded Coolermaster case! I think it was the Coolermaster CM 690 which is small compared to the 1200! One of those mounts with a chunky huge Rad, the biggest and longest you can fit is my recommendation!
Or get one of the rad's built specifically for external use would work too, they can come integrated with a pump and/or reservoir too. There's plenty of those from various manufacturers (Not Thermaltake, I repeat, NOT Thermaltake lol), such as AquaComputer.