They are 15mm, @geerlingguy pictured mounted 2 using @Zips9520 's printable design linked above, that could be useful to get the sizing right, fitting 2 x u.2 drives in a spare 3. 5" drive bay would be ideal.
You explained the cable routing much better than I did, but that’s exactly what I was thinking. Channeling is a great idea if there is space in the caddy too… If not, I think even if you could run up them up by the side of the fans, then cabling would be a lot cleaner (especially if cables connect near the back of the motherboard like on my H12SSL-i) and cooling far superior compared to the SSD caddy mounted on the back of the case.
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.