Thanks, Chaps. Your input is much appreciated. Unfortunately, where I live there is little chance that suitable commercial batteries would be available. A substitute will have to do.

I attach photos of progress to date. I also have a suitable pill box for the AAA option. However, the camera battery option is simpler, more robust and easier to reload.

I'd be very grateful if one of you could give me the final voltage specs, since that will determine whether I incorporate two or three diodes into the final assembly.
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