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.
Don't worry if the bridge isn't very good, that part can be cleaned up with a file and is hidden behind the fan.