When my Thunderbolt NVMe enclosure arrived I did what most people do and plonked it on my desk, plugged it in, and started plotting. It was running quite hot @ 62ºC. It’s a solid Aluminium enclosure that acts as a heatsink. I had a heatsink lying around, so I stuck a thermal pad to the heatsink and stuck the Thunderbolt enclosure to the thermal pad vertically, and voila, instant 5ºC NVMe heat reduction!
Most of my HDDs are Seagate expansion drives in practically sealed plastic enclosures. I have them them standing vertically on a desk with a half inch air gap between them. They run a little warm at around 45-48ºC. With summer approaching I thought maybe I could shuck the drives and MacGyver some sort of heatsink solution for them too. Just buying heatsinks is of course an option, but I looked into a cheaper DIY option and found this:
The optimal setup resulted in a 9ºC HDD heat reduction!
Rather than sit or stack the drives horizontally, I’ll stick with my vertical positioning arrangement. Aluminium U channel is reasonably cheap from an Aluminium supplier. It’s $32 AUD per 6.5 metre length here. That’s around US $1.10 per foot. I figured I need 4 lengths to do 30 drives, so around 80 feet.
140mm USB fans are another option, but I like the fact that the heatsink hack is silent, uses no power (although Aluminium manufacturing uses a shit tonne of power) and the heatsinks will never break down. For supercooling, there’s always the heatsink + fan option. 
Some rubber washers between the drives and the mounting point might help dampen vibrations.