Cut flat stock out of whatever you like. I used 5mm birch but other thicknesses will work. The layout includes an extra panel so you can choose whether or not to put a button on the top.

You will also need:

20 x Openbeam L plates
8 x 90mm openbeam
4 x 120mm openbeam
4 x openbeam feet
4 x rubber bumpers
4 x 1/8"ID, 1/4" length standoffs
1 x 2.25" panel meter
1 x 16mm button (http://adafru.it/558)
1 x Arduino Uno (or similar)
104 x M3x6 bolts
23 x M3x10 bolts
4 x M3x16 bolts
131 x M3 nuts
Hookup wire

Bolt it all together and hide an Arduino inside. Follow the directions at nycresistor.com/2012/09/17/analog-gauge/ to write up and program the microcontroller.

For my box I installed an Arduino and used the Firmata firmware to communicate with the board. There is some example code at github.com/unrepentantgeek/MeterBox as well as the most up to date Sketchup file.