I recently got into printing robot parts, but hit a snag when it came to finding mecanum wheels that could be created with almost nothing but printed parts. I looked online and found CAD files for 8 inch mecanum wheels from AndyMark, which I rescaled to ~3 inches and printed each part from there. I included the Autodesk Inventor assembly file in case you want to rescale it from there, use it in your own CAD modelling etc. My rescaled .stl's are just the right size to mount a servo horn onto, making it perfect for servo-driven robots.

Here's the site I found the CAD drawings from - http://www.andymark.com/Mecanum-s/53.htm