Vilros Heavy Duty Aluminum Alloy Self-Cooling Case for Raspberry Pi 3 Model B+

Absolutely excellent case. It's cheap (with free shipping, too), includes all the accessories needed for assembly/use, and is quite handsome as far as rPi cases go. The build quality is pretty high, too, especially for the price. It's extremely sturdy and has a very nice finish. Compared to a naked rPi when running Pi-hole in a relatively warm electronics cabinet, this case keeps the device about 10-25ºF cooler, though it's likely the disparity would be a bit larger under an actual load. I love these cases.

Great product. Well made. Should come with thermal pads instead of paste because of the gap between the case and CPU.

Sturdy construction and it keeps my Pi 3 very cool.

I was looking for a case that would provide good cooling under heavy CPU load for my 3B+. I bench-marked a bunch of cases of both passive like this one and active with small fans. This case beat them all.
The baseline test was a 3B+ with standard heatsinks installed and in a plastic case, no fan. I used the SYSBENCH tool to put the Pi under a heavy CPU load calculating prime numbers and used vcgencmd measure_temp to calculate the temperature. The baseline setup topped out at 71C (160F).
Several cases with various small fans with and without heatsinks seemed to settle out around 62C (144F). This self-cooling Aluminum case maintained the Pi 3B+ and a relatively cool temp of 55C (131F) which beat all the other cases. And for my application, less moving parts to go bad is a really good thing! Now the only question I have is when is the RPi 4 B model of this case going to be available!