IR Sensor Mount for Fan Duct

Really liked immutef's Voron carriage, but couldn't see how to attach DC42's IR Z sensor. So I added 2 tabs to the fan duct (in my slicer) to create a mount, and it works pretty well. Might be a little *too* close to the heater block, but it's been working fine for days.

For now you have to merge this stl with e3d_fan_duct.stl from the carriage. I will update to a single .stl as soon as I can generate a good one (S3D isn't exporting it correctly).

Update: I just saw that volcom8190's r