mitro had a comment here
and drd here
Only battery tubes for 26650 will fit C-size cells,
if it’s meant for 18650 only it’s too thin (~18mm)
According to Wikipedia C-size is 26.2mm dia × 50mm long
26650 is obviously 26mm dia x 65mm long (nominal, may be longer with protected cells)
so 3x 26650 should be ~195mm long, 4x C-size 200mm long
-S