This may or may not be helpful, but OCZ makes a simple metal carrier to mount 2.5" drives in 3.5" bays that can be abused to hold 2.5 inch SATA/SAS drives in the HL15 bays;
In your case you don’t have to worry about getting the drive to line up with the backplane connector, but would give you ample room to route the cables while holding the drive in the bay.and not have to 3D design or print anything.
Hot sure you coud use it to get two 15mm U.2s into one 3.5 inch bay though.
Seems like doing something above the PCIe slots or rear I/O is a better use of space though.