[Water 3.0 Series 360 mm Mounting in Tower 900]

I made the same blunder...and thanks to your posting and pic it gave me an idea!!  I removed lower cable management panel and the right side coolant pump cradle.  I then mounted the AIO cooler (Thermaltake TH420 ARGB Sync) on that rear side.  Granted, upside down (as good cooler installation standards go..) Currently monitoring temps before buttoning it up for good.  (or removing it to sink some cash into a different cooling system!!)  

    So far...so good!!