I threw together a first iteration prototype of the enclosure today. It is comprised of a 250mm section of 3in diameter galvanized air duct, a male thread 3in PVC end cap, and a 3in compression pipe plug. The current plan is to mount the power switch and led indicator lights in the white PVC end cap. Eventually, I will create a permanent water tight seal between the white end cap and duct pipe. The red compression pipe plug is meant as a service port. It forms a water tight seal with the duct pipe when tightened and can easily be loosened in order to get access to the internal electronics for troubleshooting. 

The silicone heater pads will wrap around the inside circumference of the metal duct pipe. Electronics will sit in the center core of the pipe (inside the wrapped silicone heater pads). I may route the 12V power supply connection through the duct pipe near the red compression pipe plug (with chord grips for a water tight seal).