The transformer is very cool, been wondering when someone would go a bit more 'modular,' very neat.
With those battery combos, what are the effective voltages able to run at in the different configurations? -EDIT - you did post that.
And the back endcap effectively being 'storage' is very nice, good place to put a piece of foam and a 3-5 ml bottle, perhaps?
Now....if you got 'bored.'...you could in theory make an insertable disk with a diode to limit the voltage, true it would have solder, but it could be made as literally 'pop it in to change voltage from battery sources' and just use it. Add in a 'drop in voltage reducer' to drop to 5v for those that want it, and you have 3.7v. ~5v (think maybe 4.8?), or 6v, plus possibly some other combos depending on what
batteries show up down the line.