I came across a datasheet for LT1107 switching power regulator.

Why I think it's good for this project?

- it can work in a boost configuration from just 2V in.

- it can output a negative voltage.

- it has a fixed output voltage of 12V or 5V.

- it has a built-in configurable low-battery detector.

I'm going to buy a few of these and try to build something. The typical application in a datasheet looks straightforward, so I don't even need to change it.

As a sidenote, I also found a very cool 4xAA battery holder made by Bulgin. It's quite expensive, but can be conveniently installed in a DIY enclosure.