Pi4 cooling HAT
If you want to use your RPi without display attached to the top, you might want to install this cooler HAT. Especially the Pi4 performs much better with active cooling.
The Cooler HAT provides a mount for a standard 30x30x10mm fan. In case you want to use a 30x30x8mm fan, print out the spacer. Make sure that your fan runs on 5V. Intake of the fan should be from below, the air should exit through the top.
Use a 2-pin (2 wire) fan and connect the (+) line to pin 4 of the GPIO, the (-) line to pin 6.
There are four versions of the bottom. Two for use with the Pi2/Pi3 and two for use with the Pi4. There's each a version with an additional slot for use with a Raspberry Pi camera.
There's only one version of the top lid and a spacer.