Simpler fanduct for wanhao duplicator i3 (v2.1)

First release of my fanduct for wanhao duplicator i3.
I'll upload a customizable .blend when releasing the final version.

I wasn't happy with existing fanducts because:

- required screws
- complex to setup/needs additional spacers/etc...
- hard to print

So I decided to make my own, easy to install and not so hard to print.
You can still use two screws to really make it tight.

It works well for me but I don't have much experience on fanducts.