Stackable tray for 3.5" and 2.5" HDDs or SSDs, remixed from https://www.thingiverse.com/thing:4692256 to add:

Bottom Mounting Holes

Allows both mounting the drives via the bottom screws, but also allows mounting the tray to a computer case via the bottom hdd mount points (many cases allow you to secure a hdd to the floor of the case via screws through the bottom, this allows you to mount the tray to the bottom of the case, thereby securing the whole stack).

Bottom Stack Spacer / 2.5" Tray

To go along with the bottom mounting holes (above) there's a separate bottom spacer tray (HardDriveTray-Stackable-Spacer.stl) that can be added to cases with side lips, to raise the stack above the lip so the cables can more easily fit, and it's easier to access the connectors. This tray also contains M3 nut holders (use a hot soldering iron to melt them into the print for a secure hold) and can fit a 2.5" drive in it (or you can just print a bunch of them for a 2.5" drive stack).

More Space Between Drives

Slightly increased the height of the supporting columns to get more airflow between 3.5" drives.

Cable Cutout

More easily fit 90 degree cables via a cutout on the bottom.

Top Stack Version

A version for the top of the stack without the supporting columns (HardDriveTray-Stackable-Top.stl) to fit into tighter spaces and to save plastic if desired.