This is a customizable adapter to convert a round door knob into a door handle with a lever. It is a remix of my previous design, but adapted to be overall thicker and sturdier, to render 10 times faster, and to work with Customizer.
As before this is a smooth ergonomic design, so depending on your computer it may take a few minutes to render.
The most elegant resulting appearance is obtained if you match the nom_knob_diam variable in the .scad file to within at least 0.3mm of the correct diameter for your door knob. Calipers and good calibration are helpful for this, otherwise you may have to adjust and reprint. You can also try terminating a test print early, and using the first few layers to check your measurement. At the ideal size the nut and bolt sides will clamp together tightly right as it reaches the desired grip on the door knob. If you aren't getting enough grip to your door knob, try adding a very thin strip of poster putty around the door knob before clamping the lever on.
The example stl file is for a 54.9mm diameter door knob, and is designed to be held on with an M5x18 bolt and an M5 nut.
Note: The OpenSCAD file uses my threading library for creating the properly sized recessed bolt and nut holes. http://www.thingiverse.com/thing:1686322 The customizable version has the dependencies copied and pasted in since Customizer cannot yet load libraries.