Variable resistor, 20 ohm, 3 W
To make the variable resistor unit you need
Variable resistor, 20Ω, 3W (Maplin FX40T)
Socket, 4 mm, white (JPR 705-210)
ABS knob, white (Maplin NC63T)
Plastic downpipe, piece of square section, sawn in half (DIY store)
Construct the unit as shown in the diagram.