Robin's ultimaker fan duct

I added longer/ more attachment points since previous version drooped down on the fan side and ruined otherwise fine prints that way.

I made slots for the nuts so they don't fall and hide under the table when you open up the extruder head to clear a plug or whatever...

I also added a shovel on one end which is intended to be used to shove small robust prints off the printing surface, so you can print another one...
(this is untested)