This model is for a stackable drive caddy that fits 2.5" drives, be they SSD, HDD, or U2. 

The model prints with no supports, and fit onto 2.5" drives without screws. You can then stack the caddys/drives for organization. 

Included are 4 STL.
"Hex" vs "solid" are for whether they have the hex cutouts or not (I prefer solid).
"Aggressive" vs "relaxed" is for how snug the snap/stack fit is. I found that at default draft settings in PLA, I could get the "aggressive" versions to come apart from stacking without breaking only if I was careful. The relaxed fit don't hold together as tightly, but pull apart much easier. It really comes down to your use case, and specific printer/filament. You could also adjust x-y hole compensation to suite.