The W4P Polish military metal detector was powered by a 3R12U tin-carbon battery that is not available in retail, but it can be substituted with 3 AA batteries in series.
To keep the batteries together I used 2 pieces of plastic tubing (leftover electrical conduit), both 142mm long, the outer one a 20mm diameter, and a 16mm diameter one that I split lengthwise to keep the AA batteries together through tension.
I added some holes to reduce the tension to a snug fit but not too hard to insert and remove the batteries.
The resulting compound battery doesn’t have the same length as a 3R12U, so I added a spring from a D-cell flashlight (these are available as spare parts for brands like Maglite).
Note: the cap side is -, the + side of the battery goes in first.