How about some good old fans? I had a little NAS box in my garage for a few years that ran pretty hot and I was afraid of straining the hard disks. I ended up just using lm_sensors on the server to monitor temperature, and when it got it hot, kick off an SSH command to a raspberry pi sitting nearby to tell it to turn on an exterior fan (gpio -> mosfet -> relay) I had mounted to the side of the cabinet a few inches away from the server box (literally just an old PC fan).

Super ghetto but actually pretty efficient at bringing the server's temperature down.