Replacement top piece for the NES Raspberry Pi 2/3 case by Bocklar which includes a space to mount a 30mm fan.
I used this fan from Amazon: https://amazon.com/gp/product/B07QJ4MS2L
Stress-testing using stressberry I found that the case w/ fan was consistently 20C cooler than the original case. CPU Throttling behavior was also dramatically improved. Test was done using a Raspberry Pi 3 Model B+ following the guide here: Using the testing guide found here: https://core-electronics.com.au/tutorials/how-to-stress-test-temperature-on-raspberry-pi.html
The author marked this model as their own original creation. Imported from Thingiverse.