For a project of mine I designed couple of encoder knobs for EC11 rotary encoders, here are the specifications;

• For 20mm EC11 shaft lenght I made 13mm shaft slot
• Shaft/Post diameter is 3mm
• Knob diameter is 19mm
• Knob height is 20mm
• There are two different texture along with 3 different top surface;
  • Knurled, Slit
  • Flat, Champfered, Smooth

However I uploaded step fil;e so that you can modify based on your need.