Based on http://www.youmagine.com/designs/ultimaker-2-knob--2

I redesinged fillets a bit and made it a little smaller in diameter because original design turned out to turn a bit strong, at least in my Ultimaker 2.

Printed with "Innofil Ice Blue" transparent filament.
For slicing I used Simplify3D to be able to make solid infills concentric. That way the glow effect looks much nicer than with rectilinear infill.
Other settings so that there is no infill in the part itself.

To get out the original knob I used a small screw driver, there is a small hole on the inner side of the knob surrounding, see picture.
Then push the knob out carefully. Push it a bit, then turn it 180 degrees so it does not jam until it comes out.