Kossel fan duct and IR sensor mount for J-head hot end

This is remix of Think3DPrint3D's J-head Kossel fan duct for a 40mm fan. I added a cutout and screw hole for mounting my Mini Differential IR height sensor board, and I closed off the bottom to stop the fan blowing air around the nozzle so that I can print ABS. See https://www.youtube.com/watch?v=xVrRGaeOiKA to see it in use.

**Note**: This design is for sensor boards shipped before July 2015. Later boards are slightly shorter and will need an amended design.