Knob for a ball valve. The valve shaft is 8mm in diameter, with a 6mm thick flat. Heigth of the shaft is abt. 3.5mm. 

The knob is 30mm long and 12mm high. To prevent backlash, it is important to get the flat accurate. The flat inner fitting is 6.15mm, making it a snug fit to the metal stub/shaft with my printer. The inner dia is 8.2, so it is a loose fit and should not be in the way. 

There are two variants of the knob, symmetric and asymmetric.

You need an M4x12 for fixing. The length needs to be rather accurate, as the threading is short in the stub.

PLA works for cold water valve, but ABS or similar is needed for hot water.