It looks like the extender can be drilled out to facilitate a CR123A/16340 plus 15mm spacer setup, can you please verify this?
I rather take out a spacer to switch to AA than add an extender in case of an emergency.
And many thanks for the great review, keep up the good work.
I can't confirm whether or not that would work, I don't have the equipment or skills to try it, but there does appear to be enough extra material there. My guess would be that you could drill out the extra couple of mm diameter and still have plenty of material there to keep it structurally sound. Because of the way the reverse-polarity design is set up in the head, you might find you need something around the sides of your AA to keep it held in the center to make a solid connection, but it's hard to say without trying it.