I ordered another new fan splitter from AliExpress, it arrived last week so I finally got around to modifying my DS1815+. I had an old AMD CPU cooler in the garage (70mm*70mm*15mm), I may order a new one though as the bearing sounds like its on its way out.
To install the fan you need to remove the top of the case and back fans, the fan slides down snugly in the gap once the PSU and another cable are unplugged.
I tested the temperatures with the fans set to quiet mode and used Folding at Home to load up the CPU.
Idle no fan = 50c
Load no fan = 58c
Idle with fan = 44c
Load with fan = 48c
So idle not a massive difference but 10c off my load temps, not that they were hot to begin with!
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It looks like you have also replaced the case fans - what did you use to replace them?