OPNsense for R&S GP T-10
What? I recently got some old firewalls - the “Rhode & Schwarz GP T-10” (they are even still oficially supported by the latest software release by the manufacturer!). These are embedded devices build to be deployed in workshops (or any other harsh environment) and are therefore fanless designed. They are also very small and have a very low power consumption. The hardware is quite old, but still more or less capable for a firewall.