If you need to check the precision of your voltmeter or calibrate your oscilloscope this little precision voltage reference is perfect. You find it on Ebay, Amazon, Aliexpress and so on.

I had a slight problem buying the battery intended for it and used a for me easier to find "23A" size battery. To make this slightly smaller battery stay in place I did this mounting plate. Just bend the battery connections out a bit to get to the right distance for the smaller battery.

I did not make a complete housing, just a mounting plate, since you anyway need to get to the power switch, the connectors and the jumpers to change the voltage.