Quite right, Mark. The brushes and track are exposed on the top. They
will accept a shaft, which doesn't need to be insulating as the collars
are lined with thick bakelite, but they need an enclosure as well as a
knob and shaft. I've put them inside a powder-coated electrical box for
now, and only adjust them when the power is switched off, but this is a
nuisance. And as I said, I had my eye on these two for building into a
power supply...

Best wishes,

Chris