This is a modification of Patrick Verloo's wonderful fan duct design (http://www.thingiverse.com/thing:2118768). I modified it to use the common 40x40x20mm fan ducts that you can purchase just about anywhere. There is a slight reduction in X-axis build space (see UPDATE text below).
Fan is attached using two M3x16mm screws with nuts. If you don't have them on hand, they are available at Home Depot in the drawers in the "Screws and Bolts" section, or probably any local hardware store.
Here is a video a bridging stress test I made to test it out: https://www.youtube.com/watch?v=Ip0M0nMjSOs
I'll try to keep it updated as Patrick continues to tweak his design.
UPDATE 2017-05-01: New STL files have been uploaded and I've included the STEP files. I've cleaned up the STLs by taking Patrick's suggestion on using the STP files and added a modified version. Here is an explanation of the 2 files:
Monoprice Maker Ultimate / Wanhao Duplicator 6