I have designed a housing for a digital voltmeter from AliExpress, it fits really snug and I've added a small cut out for the cable.  The first version is open at the bottom and allows you too glue it into place.  The second version Has a closed bottom with screw holes which allows you to screw it in place.

DC4.5V-30V Waterproof Digital Voltmeter Car Boat Motorcycle 0.56inch LED Display Volt Voltage Meter Mini Tester Panel Ammeter - AliExpress 34