Biggest problem I had with passive cooling systems is having a case that flows bottom to top to help flow. Adding a 70-100cm 'chimney' same size as top vent helps significantly.
Indeed, PSUs that run fanless at low wattage are great.
There have been a few attempts to make heatsink cases, like Zalman TNN 500AF. Thermalright did several passive cooling concepts back in 2005-2008, one in pic below.
Fanlesstec does passive cooling products.
https://www.fanlesstech.com/2018/10/passively-cooled-pc-with-65w-cpu-47w.html
Indeed, PSUs that run fanless at low wattage are great.
There have been a few attempts to make heatsink cases, like Zalman TNN 500AF. Thermalright did several passive cooling concepts back in 2005-2008, one in pic below.
Fanlesstec does passive cooling products.
https://www.fanlesstech.com/2018/10/passively-cooled-pc-with-65w-cpu-47w.html