Just an idea–I don’t have a design for the PCB.
Some elements to include:
–“bulls-eye” circular end, with active copper only in the middle, a bit less in diameter than a AA cell, but (isolated) copper or silk screen circle as guides for C size, and the overall PCB the diameter of a D cell, so one could cut away the excess with scissors to match whatever size is needed without shorting the layers, but still have the pcb diameter help center the contact
–pads for through-hole 3.5mm jack (I’m finding stereo ones are a bit cheaper/easier to find than mono)
–holes/pads for directly soldering wires (might be shared with the holes for the jack): larger holes for insulated wire to pass through for strain relief, smaller holes/pads for soldering the stripped wire
–neck down the width between the ends to make it easier to route in the battery compartment
What layout software do you use? Would KiCAD work?