This is a customizable adapter to convert a round door knob into a door handle with a lever.

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.

This is designed to be held on with an M5x18 bolt and an M5 nut.

Credit: rcolyer