Yoctopuce Yocto - Volt
This sensor device is a digital USB voltmeter that allows you to measure voltage automatically. It can provide quite precise digital measures for low DC voltage (0.01 Volt precision), and also works for higher voltage including AC, up to 250V (RMS), with a precision of 3%.
The sensor device is isolated: the sensing part is electrically disconnected from the USB part. This means you can measure any voltage difference between -250V and 250V, without risk of frying your hardware.
You can use this long, thick enclosure for housing this sensor.