You can always buy a aftermarket aluminum case from Silverstone or something and just swap the motherboard and I/O insert across from the Deskmini if you don't like the steel case it ships in. Also I don't think any of these cases are going to heat sink the components inside in the way you seem to think they will. In order for that to occur you're going to need to have some degree of direct physical contact between the hot surface (IHS) and the surrounding case. The major component for cooling in a mini STX based system like this is direct and reliable access to fresh air from outside the case (ie. adequate and appropriately designed for airflow given your choice of cooler.
Also if you're really keen on a passively cooled solution, you could always get one of these NUCs
http://www.logicsupply.com/products/industrial-computers/ml100-series/
Also if you're really keen on a passively cooled solution, you could always get one of these NUCs
http://www.logicsupply.com/products/industrial-computers/ml100-series/