Make the body an empty can at 52mm or so. Keep the same contact ring on the head/driver. All parallel, so who really cares how they’re situated?
A cheap “speedloader” that’s got a cross or trefoil cross-section in the middle to keep the 4 18650s or 3 21700s upright, and a bottom-with-springs that would have the springs make contact with the cells on one end and with the body on the other end.
One for 3×21700 (shorter springs) and one for 4×18650 (longer springs).
Nb: I don’t mean a full cell-holder or carriage that’d make the assembly any wider, but just a disc-with-springs at the bottom and central spacer/form to just keep the cells upright. Kinda like a big-ass thumbtack but with cross/trefoil pointy-part.
Then you can have the same light take both configs, 3 or 4 cells.
Eg, if I’m remembering right, the SD01 with its 1pc body, has the cloverleaf-shaped insert-with-springs just drop in and snap-fit to stay, vs the Q8 where the cap unscrews and the PCB-with-springs is hard-mounted inside. The former is so much simpler and cheaper, and works just as well if not better (loses both threads and O-rings, as well as another potential for water-ingress).