This is a dual 18650 case, originally designed by tekhertz. I modified the design to mount a circuit board on top, and a cover for the circuitry. Aside from making the end-caps taller to meet the circuit cover, I modified them to include built-in faux supports, so the caps should be printed at 0.2mm layers without support material. After printing, the single layer can easily be punched through with a file or drill.
2 caps are available. One without any holes, and a second with a cutout sized for 2 USB ports side by side. Although the photo shows cutouts in the circuit cover, these are not built into the model as they are specific to my circuit. I recommend that any cutouts needed for buttons or switches be cut out of the printed model instead of modifying the model if possible.
End caps attach to the core with M3 screws and nuts. I found that M3x8 screws worked well for the battery posts when paired with half of the spring from a clicky pen attached to one side.
The circuit cover and circuit board mounting holes are designed for M2.5x4 screws.
The end caps have a channel cut in to connect the ends of the batteries. For running the wires from the batteries up to the circuit, I filed 2 small grooves into one of the core for wires to pass through. This is the end where I attached the springs to the M3 screws and consider this end essentially permanently attached, only removing the other, free end, to change the batteries.