Here is a stackable HDD case, suitable for 2,5'' hard drives, (12mm thickness or less).

No screws needed, each hard drive stays in place due to the friction with case walls. I uploaded the original FreeCAD file, feel free to improve the design !

I just stacked 3 of them, I'm sure you can stack 2 or 3 cases more, with the applied offset. The offset make it easier the SATA-to-USB bridge I use to connect the hard drives.

Even if FreeCAD is a good parametric 3D software design, I had to repair the final model (Mesh mode in FreeCAD). STL file repaired provided.

Enjoy !