Parts needed:

- 7mm Socket (outer diameter 12mm)

- M4x20mm Machine Head Screw

- M4 nut (Standard - NOT Nyloc)

Drop the nut into the socket, and press the socket into the knob. Insert the screw from the back of the knob and fasten it to the inserted nut. I suggest adding a drop of loctite.

Print parameters don't matter. 

I printed mine out of Polylite ABS, 0.15mm Layer Height, 25% Cubic Infill.

STL and .STEP files included.