This is an adapter to convert a round door knob to a door handle with a lever, inspired by the design by tbirdrv. I redid this concept from scratch in OpenSCAD so that the dimensions can be fully parameterized to fit various door knob sizes with a close match so that the pieces line up.
I am fond of smooth ergonomic 3D printed parts, so this design calculates a curved hand-friendly structure with smooth rounded edges. This results in additional calculation time for OpenSCAD to render the final result. If you turn the sfn parameter down to 0, then it will render quickly for adjusting dimensions. I recommend raising it back to 4 for the final render for a smooth result.
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.
The example stl file is for a 54.8mm diameter door knob, and is designed to be held on with an M5x12 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
Installation tip: If you printed just a tiny amount too wide, or you want to increase the grip on the door knob, then try stretching a very thin strip of standard poster putty around the door knob before mounting the lever around it. This will significantly increase the coefficient of friction between the two surfaces, in a removable way, and give it a sturdy hold without needing to excessively tighten the lever around the knob.
0.5 x/y, 0.2 z, 1.5 perimeter, 1.0 top/bottom
Using solid infill can provide added strength for handling the leverage that can be applied to this part when installed on a door. Also if you use PLA, and sometimes print a little low in temperature, increase your temperature a little (at least 210C) for this print to avoid delamination under the strains this is normally used at.
The design is symmetric, so left and right side door knobs work with the same stl file. Slightly better aesthetics are obtained by orienting the design so that the side which prints down will face toward the door.