Evolv's original Kick was designed to use a single battery only. Since the Sigelei version is a copy of Evolv's basic design, I doubt it would reliable handle more than 4.5 volts or so on a constant basis. There are warnings against stacking batteries with the Kick, and I believe it voids the warranty, so going above 4.2 volts is probably not a good idea.
Most LM2596 step-down transformers have a mechanical adjustment, have you looked here?
lm2596 | eBay Most of them are from Hong Kong/China, but the ones I've bought and used were sold/shipped from the U.S.