Bi-Directional 40mm-50mm Fan Adapter
Description
These two sizes are confused frequently enough that it warrants an adapter. I think it's because the 50mm fans have 40mm mounting hole spacing.
Instructions
I don't have a 40mm fan here at the moment, so I'm not 100% sure about the hole spacing, but I took measurements off of a printed part made for 40mm fans, so it should be very close. Please comment if you find it does not match, and I will make the corrections.
License
Bi-Directional 40mm-50mm Fan Adapter by wildseyed is licensed under the Attribution - Share Alike - Creative Commons license.

It would be a perfect solution to the problem I'm having, if it could match a 120mm fan. (I just needed an outter case-buffer to set my fan on an open vent). I could not get your .stl file to open in sketchup or OpenSCAD, do you have the .SCAD file or a 120mm version? :)
Give me the two sizes you want to mate, and I'll post it here, and include a format you can edit, if that is possible. 120mm Fant to ?