After building my bench variac, I realized that having something I can move around as a source of variable AC would be useful.

I found another Porwerstat Type 21 on Ebay that came with a knob, and it fit in a Carlon 6"x6"x4" junction box with just enough room for another Chinese AC voltmeter, small smps, an AC outlet, power switch, and banana jacks / binding to round out the build.

Because AC varies, rather than label the scale with actual volts I chose percentage of the incoming voltage. Finally, 'Spannungsprozentsatz' is voltage percentage in German - because everything sounds more impressive in German. ;)