for those who are interested, I get this CNC metal shell for RPI4:
https://www.aliexpress.com/item/4001309643679.htmlIt looks very cool and small.
RPI4 running at arm_freq=2100 and gpu_freq=700 keep temperature at about 40-50 °C with usual load.
Here is a small test with stress -c 4 (continuous full CPU load):
05 min = 52°C
10 min = 57°C
15 min = 61°C
20 min = 63°C
25 min = 64°C
30 min = 67°C
It don't exceed throttling with no fan.
The only con is that WiFi signals are reduced for 20 dB and it don't see access points with weak signals on a long distance.
The speedtest.net shows that the speed drops down from 43 Mbps to 20 Mbps when connected to nearby WiFi router.
PS: When I put a little passive heatsink from an old 1990's ~20W CPU (60x55x50mm with no thermo-interface just touch) on the top of this metal shell it keeps temperature below 50°C even with high CPU load