I would be very interested in what you come up with, for a conversion kit for the 6D. I was kind of thinking of using a pair of 26650 batteries for it (and spacers/sleeves, or whatever), but am completely open to 18650 use, as well. I've gone with Tenergy Centura D NiMs for both of my 2D lights, and they're working out well. They're car lights (that don't see a lot of use, either, with on-person or bag-carried EDC lights being used, first).
Unlike Mr. Fixer, I do not like the idea of using proprietary batteries, or using battery packs in a flashlight. It's kind of a deal breaker, for me. I think USB charging is way too slow, and I prefer to have individual access to the battereis of multi battery lights, and being able to charge them individually, at will, whenever I please, immediately replacing the depleted batteries with fully charge ones, in the meantime. I have plenty of decent chargers, so that's not an issue.
This is the only light I have left that's still running on the dreaded, under-performing alkaline batteries. I hate alkalines with a passion (and haven't bought any, in years), and I almost never use this particular light (since it was replaced by numerous 18650 lights; but it would be nice to use it with more serious batteries, even if only from time to time.
At the moment, it's fitted with a TerraLux 1000 (LED) lumen conversion module that my folks gave me, several years ago. I subsequently replaced that cheesy plastic lens with a nice Borofloat lens (No more plastic for me, folks).
I will look forward to seeing your future pictures and posts! Welcome to the forums