This is a case for a mini voltage display. It fits Amazon product # B079N98PY4 described as:

HiLetgo 5pcs Voltage Meter 3 Wire 0.36" DC 0~30V Digital Voltmeter 
Gauge Tester Red LED Display Panel Mount Car Motorcycle Battery Monitor Led Voltage Display

They cost about $9.00 for a pack of 5. Be careful! There is another similar volt meter that comes in a pack of 5 but it has multiple colors and 2 wires from each. The one you want is the three wire red display models. These are better quality by far and also have an adjustment pot on the bottom if you need to adjust it for accuracy.

This model snaps together and does not require any screws. Use double stick tape to stick it to wherever it's needed. I use it on a battery in a go kit for amateur radio.

Printed using PETG with no supports needed.

Model designed in OpenSCAD which is a free download. Source is included here.