My son was designing a model plane that flies on a string.  He needed a battery pack that fit into a 24mm circular cavity. I really liked msatbsx's  modular design, it is great and flexible to work with. However it was a little too big to fit in the plane's cavity. So I trimmed a single cell to what I figured was the minimum, did some mirroring and it worked like a charm.  It might be of use to someone else who is a space constrained.