If I commissioned a machine shop to do it the switch will be the hard part. I would use one cheap off the shelf component, not ideal but ok. The cheap off the shelf component would be just use existing short cigalike batteries and treat them like a consumable part. It would have a tiny rebuildable atty similar to the Sophia but without the same
juice flow and a very tiny steel
tank.
The tiny rebuildable atty has been done in the Diver and the Sophia, the
tank is just a section of threaded steel tube and would be very small, maybe .5 ml capacity. The user serviceable auto switch could be the real problem. There are plenty of auto switches to look at but they are of lower quality and designing one that was meant to be user serviceable might be hard for a one off.