Did not know about Tinkercad - it is really not too complicated to use. I played around with it a little and could personalize the knobs, i.e. engrave my name on the top :-)
I had my knobs printed with "glow in the dark" pla. On 3dhubs.com a knob costs me around 3 EUR here in Germany, pla or abs. Do you think abs is a better choice?