Modular 18650 battery holder / cap

Just add a wire through the two small holes….

…And pretty much done. I secured the wires with some hot glue.

The caps can be fixed to the cells with either a zip tie (for more permanent setups) or with some kind of elastic (I used 4mm rubber cord, for easy exchange of a cell).

I crimped some DuPont connectors to it, making it nice and modular for e.g. a TP4056 charger module or a powerbank module.