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Put the x-carriage-fanmod.scad file in your Mendel 90 scad folder, open it in OpenSCAD and compile (Design -> Compile and Render CGAL) then export STL (File -> Export -> Export as STL).
This will also get you the correct teeth for the belt you're using, rather than the 6-T5 belt I have.
Because the LM12UU bearings are bigger, I'm not sure if the fan will fit as well, but go ahead and try.