With the new Slic3r "Cooling" option, using a fan in the hot-end to cool the layers faster is a great improvement in the print quality and speed.
This part was designed to make it possible to cool down the hot-end heatsink and at the same time blow some air on the printed part, without dissipating heat from the hot-end itself.
I was very inspired by thing http://www.thingiverse.com/thing:16266 but also wanted to have airflow over the printed part.
This design was made using Sketchup, I will learn OpenScad soon and redesign it, I promise.
Use the .stl provided for the J-Head hot-end. It could work for others, but I encourage you to adapt the dimensions to your hot-end.
You can cover the plastic part that is closer to the nozzle with some kapton tape.
I have printed lots of things with this dual airflow system and it is working really well so far.