update 25/11/06: I added an SCAD file for the slim version to easily adjust the arm length and hole spacing.

I didn’t have a ring binder or any other kind of binder at home. The one I found on Thingiverse by ckpinetree didn’t fit, as the holes have a 70mm distance. I took the concept and drew my own in FreeCAD with 80mm distance. The file is included in case you want to change anything.

There are slim versions available that can be printed in just a few minutes, and larger ones with a plate for added stability. Both versions come with three different ring lengths: ⌀80mm, ⌀60mm, and ⌀40mm.