Update: 21st July 2023
I have updated both bases to have a matching pattern for the back as well as making the front sticky-out bits more aesthetically pleasing when stacked. The original files have been renamed with the prefix "(old)".

This is a stackable tray to hold full-size hard drives. The tray has four holes in the base and three holes in each side for screwing the HDD into the tray. There are another four holes to screw in an SSD in place instead.

When stacked three-high, there are holes in the side for mounting a 120mm fan.

A separate base is included with four holes for screwing down into a permanent position, as well as a lid to top it all off.

If you want to lock the trays together to make them more sturdy, you can feed a short length of 1.75mm filament through the passthrough holes in each post (see photo). When it's time to separate them again you can pull the filament out (if you left a small amount hanging out) or poke another length into the hole to push the locking filament out the other side.
If you find it a bit tight, just run a small drill through it until it fits easily.

Version 2 (v2) is identical but has a smaller hexagon pattern in the base for those who find it more visually pleasing.

A smaller version specifically for SSDs and includes a separate lid can be found here ... https://www.thingiverse.com/thing:6114918

An alternative version with the SATA ports facing "out" can be found here ... https://www.thingiverse.com/thing:6133437

I printed mine with the following settings...
... Material: PLA
... Layer height: 0.2mm
... walls: 2
... Infill: 10%
... Rafts: None
... Supports: None (I placed a single support in the centre of the SATA port slot, though this shouldn't be needed)