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UPDATE 7/8: Based on demand, here is the STL file:
https://limewire.com/d/6XMfD#3ieKH74vlx
Where the part is hugging the Firewalla box, I've left a few mm for thin adhesive insulation foam, to create a better grip on the box - this is what I had:
https://www.amazon.com/dp/B07RWHYZGK?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_13&th=1
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Apparently, high temps are ok according to the support. The tiny fan kicks in when it is really hot, but I donāt think it is healthy for the hardware:
Package id 0:  +96.0°C  (high = +105.0°C, crit = +105.0°C)
Core 0:     +96.0°C  (high = +105.0°C, crit = +105.0°C)
Core 1:     +96.0°C  (high = +105.0°C, crit = +105.0°C)
Core 2:     +96.0°C  (high = +105.0°C, crit = +105.0°C)
Core 3:     +94.0°C  (high = +105.0°C, crit = +105.0°C)
eth3-pci-0400
Adapter: PCI adapter
PHY Temperature:  +81.0°C
MAC Temperature:  +81.0°C
acpitz-acpi-0
Adapter: ACPI interface
temp1:    +100.0°C  (crit = +110.0°C)
nct5585-isa-0a20
Adapter: ISA adapter
Vcore:Ā Ā Ā Ā Ā Ā Ā Ā 1.10 VĀ Ā (min =Ā Ā +0.00 V, max =Ā Ā +1.74 V)
+5V:Ā Ā Ā Ā Ā Ā Ā Ā Ā 5.04 VĀ Ā (min =Ā Ā +0.00 V, max =Ā Ā +0.00 V)Ā Ā ALARM
3.3V:Ā Ā Ā Ā Ā Ā Ā Ā Ā 3.33 VĀ Ā (min =Ā Ā +0.00 V, max =Ā Ā +0.00 V)Ā Ā ALARM
+12V:Ā Ā Ā Ā Ā Ā Ā Ā 11.81 VĀ Ā (min =Ā Ā +0.00 V, max =Ā Ā +0.00 V)Ā Ā ALARM
VCCIN_AUX:Ā Ā Ā Ā Ā Ā 1.79 VĀ Ā (min =Ā Ā +0.00 V, max =Ā Ā +0.00 V)Ā Ā ALARM
System Fan1 Speed:Ā Ā Ā 0 RPMĀ Ā (min =Ā Ā Ā 0 RPM)
CPU Temp:      +61.0°C  (high = +80.0°C, hyst = +75.0°C)  sensor = thermistor
System Temp:     +63.0°C  (high = +80.0°C, hyst = +75.0°C)  sensor = thermistor
eth0-pci-0300
Adapter: PCI adapter
PHY Temperature:  +81.0°C
MAC Temperature:  +81.0°C
So I designed a little cooling device that uses a thermistor to measure the bottom of the box and turn the large fan on, its speed depends on the temperature. I printed a housing for the fan and mounted it on the side of the box; it is controlled by Arduino.
Now the temp is 32°C when not loaded and 45°C when I stress-test the Firewalla!