[ANET A2] Fan Duct

This is designed to work with my [Fan Holder](http://www.thingiverse.com/thing:2235382) print, I'd reccomend you use that with this (Since it's specially designed for 50 mm x 50 mm x 15 mm fan designs, and my fan mount works with that size of a fan.) print. I'll upload a better design later, but for now, this is the best I can do. Thanks.