Step 1: Material :

You will need what there is on the picture plus a power cable 19V DC

voltmeter:

LM317 (2pcs)

10K potentiometer

prototype board (20pcs)

Alligators clamps

box

These links are mostly amazon products so you can find cheaper on other websites (banggood, aliexpress, ebay, ...).

(the other pieces are from recycling so there is no link to find the exact same)

Step 2: Circuit :

The power supply is based on the LM317.

I modified the basic circuit of the LM317 to add switches and a voltmeter,

folow this schematic and weld the components to the board.

The "switch 1" is the black one, it is the power switch.
The "switch 2" is the lever when it's in "OFF" position the voltmeter displays the voltage and you still can adjust it, but no current flows. However, when it's "ON" current flows and the led turns on.

The voltmeter has 3 wires: red, black and yellow (or white),
red and black are the wires which gives current to the LEDs of the display. So black goes to ground and red goes to VCC (after the power switch).

The Yellow one is the wire which measure voltage it's why I place it at pin 2 of the LM317 (OUT)

You can add a 10µF capacitor between out and ground for a better stability

Step 3: Drilling :

Draw gide-lines on the box and drill some little holes around the emplacements of the voltmeter and the switch with a drill-press, then with a hammer, break the center like on the picture and sand the edges to allows voltmeter to fit in.