oh brilliant, thank you! I wonder where I went wrong:

ID: 1.25"
18650 battery diameter: 19mm
CF cross section: 0.93" x 0.30"

When I try to fit a 9mm diameter circle and a 0.93" x 0.30" rectangle inside a 1.25" diameter circle, it doesn't quite fit. I guess because the board isn't a solid rectangle, so therefore the chassis parts just slip on the ends?