This enclosure for those common cheap mini voltmeters lets you clip them into the power rail of your breadboard.

Step 1: Solder two wires to the board (combine red+yellow to a single wire, this way you don't need a separate power line, downside is you won't be able to measure anything below 4V)

Step 2: Solder those wires to a pin header.

Step 3: Place the voltmeter in the enclosure bottom. (Omg, what a mediocre solder job. Good that nobody will see it once it's enclosed.)

Step 4: Put the enclosure top on and push the mounting clips into their holes until they snap in with a satisfying click sound. 

If it's too loose/tight, you can scale the clips in your slicer.

Done :)