Proper on-off-on switch wiring advice
Most elegant solution is probably to use a 3P3T toggle or slide switch such as this 4P3T one from e-switch: 



Use one pole to control the connection of meter to either the voltmeter lead or the DC-DC output (as you show it), a second pole to control power to the boost converter ('on' only in one position), and a third pole to control power to the DVM ('on' except center position). In the case of the slide switch that leaves one pole spare. 

That allows the boost converter to be powered down when only using the voltmeter, saving some battery life (and preventing it from outputting voltage on the output lead when it is not desired). 

As the others have suggested use an 18650 with full internal battery protection PCB using MOSFETs and an IC (they're a few mm longer) not an unprotected cell.