You should try checking out r/3dprintmything. I got my G-Wii case and it's button supports printed and shipped for $20. Of course it won't be shapeways grade prints, but it's pretty good for it's competitive pricing.
This is what I ordered, you probably need to adjust the component's quantities.
https://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=a201a8ad17
Sorry for being a lazy person, but would this solution cost less than a bms and pth regulators?
Also, isn't this like the powermii lite?
EDIT: oops I'm stupid and should have read closer to other responses ignore this
Just saying if the guides here all say to use a 18650 battery, there's probably a good reason to use a 18650. Also, do you really want to keep swapping out your batteries after each play? Side note, alkaline AAA and AA actually have less battery capacity than the 18650s do. Don't know too much...