I got a bit frustrated finding a knob to fit the 6mm knobs on the uBitx.? Fortunately I found these adapter sleeves to bring the shaft OD up to 1/4 inch so I can use standard American 1/4 shaft knobs!??? I received them today and they work.? You do have to be careful to pull them out slightly before you tighten the set screws so the knob has enough clearance to be pushed in for the switch to work.
https://www.ebay.com/itm/222983860788
-Jonathan
Jonathan