Ah...but there's an idea for everything it seems. https://www.youtube.com/watch?v=_E1yP5NV9NY
As I mentioned, I did that with my 18v Ryobi batteries.
I used one of the old blue Ryobi flashlights. With a little sanding, to make the parts fit precisely one inside the other, it looks like I never did anything to it.
Had a lot of fun with my boss. I convinced him for a while, that Ryobi 18v batteries actually fit on his Lincoln grease gun.