I was given an old CTC Generation 2 clone  of MakerBot ‘The Replicator Dual’  circa 2012 as my first 3D printer.  I didn't need or want dual extruders but I did need a cooling fan.  This bit of plastic replaces the extruder with a duct to redirect the fan which was previously used to cool the extruder, which has been removed.  It substantially improved my print quality until that printer died and I got an MK3s+ kit.

It's in pink PLA as that's what I had laying around.  No supports.

This was originally published on Thingiverse as thing 5383587, but they lost my text instructions.  It should be pretty obvious.  I also used a modified cable holder.  Designed in FreeCAD, design files provided.