After seeing the other headphone hanger designs, and finding they didn't quite fit my Ikea Vika Amon desk, I decided to build my own. In particular my design idea is based off of http://www.thingiverse.com/thing:17531 - though my design did not modify his. (please let me know if this is considered a derivative!)
The only issue I had was my original object was 40mm wide, which proved to be a bit much. I've reduced the STL to 20mm.
Download and print.
If the gap is wrong for your desk, you can resize by importing the .stl file into Blender. Create a cube and size the cube to the correct proportions. Then scale the headphone clip as needed to fit. (Or if you know more about Blender than I do, just set the size accordingly.)
I'm including the original SVG and Blender file if needed.