So I just finished the chasis system for my daughter's saber for her birthday, and I had centered it around a 18500 cell (I had 4 of them handy at the time), but they aren't exactly a common size.

It works for now, but my question is - if I decide to change the battery solution to a more common size, what should I use that will fit inside a 1" diameter PVC pipe and is not longer than 50mm? I didn't allow for any extra length in the battery (due to cram-fu restrictions).

Here is a picture of the chasis for her saber. Nothing fancy.