A new Akasa case transforms the ASUS PN50 mini-PC into a passively cooled system. The Turing A50 retails for £125.00, or €130.00 in Europe.
https://www.notebookcheck.net/Akasa-Turing-A50-A-passive-enclosure-for-the-ASUS-PN50-an-AMD-Ryzen-powered-mini-PC.524631.0.html
I've built with Akasa cases before, and whilst their build quality is very good, the problem is that they only concern themselves with removing heat from the CPU. There weren't any provisions to keep VRMs, chipsets, or NVMe drives cool. Hopefully this has changed.