Summary

Derived from Ian Johnson's fan duct:

Mount changed to accept magnets and adjustment screw to fine tune mounting angle.

Instructions

Full installation instructions can be found here:

Print duct and mount. Full credit to Ian Johnson for designing the originals.
Use a vice to push in 1/4 x 1/4 inch magnets.
Use acetone or glue to attach mount to duct.
Drill out hole on mount to accept screw of your choice. Insert screw, cutting a thread in the plastic as you go.
Insert 40mm fan and mount on Solidoodle extruder. Adjust screw to get duct parallel to bed.
Wire up fan and enjoy!