I tried several existing models but none of them really fitted to my printer so I decided to design a new one.
This is a mix of good ideas I found on other designs.
http://www.thingiverse.com/thing:259688, http://www.thingiverse.com/thing:221306 and some of my own adapted to better fit to Wanhao Duplicator 4X
You can adjust the height to really fit to your nozzle height.
Don't forget to break the small supporting part on the front.
Feel free to improve :-).
-- UPDATE 11/5/2014 --
Added inside reinforcement to Rear Fan v4.5 as suggested
-- UPDATE 5/4/2014 --
Added Rear Fan for even better results.
Rear fan can be fixed to replacement cover.
-- UPDATE 18/5/2014 --
Added Rear Fan v3.3 with clips to be more robust and resist to vibrations.
You can use 2x M3 nuts in the Rear Fan and use 2 * M3 10mm screw for attaching the clips.
-- UPDATE 25/7/2014 --
Added alternate version for Front and Rear (v4-4) + Y axis switch stopper that can avoid to have issue with the back
-- UPDATE 08/01/2015 --
Added a photo that explains how to install v4-4 (same system for Front and Rear)
Just print and remove the small supporting part on the front.
You'll need 2xM3 60mm cap screws hex head.