No offense, but that is really a dead end street.One thing to keep in mind with this type of VV is that there is no feedback so as your battery voltage drops with use you will need to turn up the VV slightly to compensate. I will add a cutoff so that it will shut down when the batteries reach a certain point however so that they can not be drained beyond a certain point.
If I was going to offer folks a higher volt option I'd go with a "5V" GLV2 approach, not VV, but a higher volt option. Still a very popular and often recommended HV PV. It's fans are very rabid and loyal.
GLV-2 Electronic Cigarette [GLV-2] - $110.00 : Electronic Cigarettes and E-Liquid - Great Lakes Vapor
Tons of reviews praising it and it's performance on YouTube.
All you would need is a tube that can handle a stacked pair of 3.7V cells (16340s) and a 1Ω power resistor (about a 3W would cover it) 'switch'.