Knob for a Whole-House-Fan or an attic fan with a 2 speed Switch.
The knob is not easily available at the big box stores. The shaft of the switch is 6.25 mm at the base with a double 'D' cutaway at the end. The smaller diameter with the cutaway D's is 5.1 mm. I included a slight 0.5 degree angle along the shaft axis of the knob for a tight press fit.
I uploaded the Fusion file in case you want to edit, and the STL and gcode if you want to print directly.