LGR Blerbs | Solid Aluminum Raspberry Pi 4 Case by DeSalvo Systems @LGRBlerbs | Uploaded July 2020 | Updated October 2024, 1 day ago.
Just bought this thing after seeing some videos about it, and yeah. The case itself is the heatsink! Looks awesome, fits together nicely, and keeps the RPi4 running smoothly with passive cooling.

Here's where I grabbed it:
desalvoinc.com/collections/frontpage/products/housing-for-raspberry-pi-model-4b