It's time for an update!
Why? Because the risk of short-circuiting with the first solution was too big and the design was not flat enough.
So here is my second prototype:
This one drops into the bottom of the Battery-tube (the "cold" side without any risk of short-circuiting).
The PCP is a doublesided 0.5mm thin, diameter is 17mm (like CR123A). The backside has to be connected to the inner ring on the topside. The center hole must be smaller than the pole in the battery-tube (< 6 mm). The resistors are SMD type 1206. I choosed 3 x 47 Ohms in parallel that gives 15.67 Ohms and 0.375 watt.
If you plan to use rechargables I would take at least 4 x 68 Ohms in parallel that gives 17 Ohms and 0.5 Watt. The resistors are also 0.5mm thin so the complete drop-in-switch is about 1mm.
jugeh