The APU2 makes a great platform for a pfSense firewall / router.  Its silent and its fanless design sinks the CPU heat to the aluminium case.  However, if you have two then the lower one gets hotter.

This 3d printed spacer fits between the two APU cases allowing air to flow between them.

I printed the one pictured in PETG as PLA would likely deform as the CPU is rated up to 80 ºC, although the case normally is normally body temperature.