I am sure like me many of your retro computing interests extends beyond NeXT.
I have designed a SCSI2SD v5.1 / v6 mount for internal 3.5" and 5.25" drive bays.
The mounts have all of the holes for a 3.5" HDD and 5.25" hard or optical disk based on their respective SFF specifications. The connector is placed as close as possible (centered, and a bit higher) to where a normal drive connector would be.
The mount holes on the adapter for the SCSI2SD use M3 screws, 4-8mm length should be fine and the side/bottom mount to drive cage holes use standard #6 case screws.
I use the 3.5" adapter in my Sun SPARCclassic and the 5.25" adapter in my NeXTcube.
5.25" adapter is available on Thingiverse here:
https://www.thingiverse.com/thing:46448943.5" adapter is available on Thingiverse here:
https://www.thingiverse.com/thing:4640954