Download the scad file from files section and generate the 3d model in openscad, or skip this step and print from the attached .stl file.

OD = 31.9;        //outer width
OH = 15;        // height of main cylinder
ID = 5;         //inside diameter for mounting hole
KD = OD+10;        //outer diameter for knob
KH = 10;         //height of knob
SD = 6;         //shaft diameter
SH = 10;        //shaft height

module knobMain (){
    cylinder(OH,d=OD);      //main cylinder
    //grip cylinder with grip on outside
    translate([0,0,OH+KH/2]) cylinder(KH,d=KD,center=true);
}

module knobCutout (){
    difference(){
        //cylinder to remove center hole
        translate([0,0,-0.1]) cylinder(h=SH,d=SD);
        //keep the tab on the switch
        #translate([SD/1.5,0,(SH/2)-0.1]) cube([SD,SD,SH], center=true); 
    }
}

module buildMain (){
    difference(){
        knobMain();
        knobCutout();
        // this cuts out the small indents around the outer edge
        for (i = [0:360/7:360]) {             rotate([0,0,i])
            translate([KD/2,0,OH+KH/2])
            cylinder(KH+0.1,d=KD-OD-2,center=true);
        }
    }
}

buildMain();

You may need to enlarge the inner hole to suit your printer and rotary switch. generate the .stl or use downloaded file. Print 3d part.

Use sand paper to smooth the corners and printing artifacts if necessary. I use a kitchen stove gas flame to smooth the plastic scuffs after sanding.

Use a black marker pen and draw a triangle to indicate the switch position if you like. See if you can do a better job with the pen than me.