I need to add a voltmeter and amperemeter to the output of the box.

The autotransformer has a small needle display for the output voltage but it's not very precise, and I'd use a current display as well but I am not sure what sort/model would be best.

I know there are ready-made modules but they feel underwhelming, a little accuracy and precision would be more than welcome...

I don't want to tear up a multimeter as I did with the #Precision current generator 

Powering the device(s) is another significant detail to keep in mind, as I don't want to worry about dead batteries, or measurement inaccuracies when the power comes from the mains itself.