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I bought a 1 Tb external drive about 6 months ago. It’s two 500 gb drives in a single box, with a controller, and a $0.60 fan.
The fan failed at some point in time. I went down to the file server one day to find that the unit was almost too hot to handle.
I replaced the fan with a $7 fan from microcenter, but in doing so, noticed a couple of things:
- It would have cost the manufacturer $2 to install a better fan
- the fan would be a heckuva lot more efficient if there wasn’t a GREAT BIG CIRCUITBOARD blocking the airflow.
- If I set the drives upright and left an airgap, the drives stay almost as cool as if I used a fan.
- If the fan works AND there’s an airgap, the drives are cold to the touch.
Not having access to the same tools the drive manufacturer has, I needed a geometrically simple solution I could make in the shop. I carved off three pieces of 7075 aluminum from the mother chunk, and got to work with the flycutter and ended up with this:
It’s stable, abstracts the distances between the mounting holes (so absolute dimensions were less important) uses a minimum of materials, and did everything I needed it to. Now the drive array is COLD, and I know that when (not if) the fan fails, the drives will happily hum along until such time as a power surge or failed bearing ends their service. In the meantime, I have a second set of backups. 
