The lithium ion cells are nominally 3.7V but in reality they are 4.2V at full charge. Voltage declines as charge declines, so when it’s at 3.7V it is depleted by about 1/2 (if I recall correctly). Two such cells in series would reach as high as 8.4V at full charge. A single cell with a metal spacer (a ‘dummy’ cell) could be used to provide only the voltage of a single such cell, but that voltage might still ruin your device. LiFePO4 chemistry cells (3V nominal, 3.6V fully charged) might be a somewhat better alternative, but I don’t know if they are made in 32650 or similar size.
I would recommend using two NiMH rechargeable D cells, such as Tenergy Centura. They would give you about 10Ah of current which is more than you’ll get out of the 32650. You might also be able to use two AA NiMH cells (Eneloops are the best) in AA-to-D adapter cases, but if your device draws too much current at one time it could make the voltage sag too much from the AA cells.