> You've reminded me of N batteries though, which I think are
> roughly half-length AA

How about two of these ?

www.rsnewzealand.com

Product code 2636033

NiMH, 12mm long x 17mm diameter, 2.4V, 80mAh

If this is a one-off, perhaps take something apart and use the
bits - like a cell-phone battery pack ?

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