It is baffling why camera gear manufacturers haven't adopted 18650 batteries yet. You can find 18650 in everything now, from Tesla cars to high power flash-lights, USB power banks, and most laptops. My friend pointed out that the average photographer doesn't like change, had to be dragged kicking and screaming to convert their workflow from film to digital, and AA is something that they understand.
Recently I tried to find evidence that hands-on DIY MacGyver type photographers are using 18650 with their flash units and cameras only to find an unconvincing adaptation of the idea, or reasons against it (this had to do with maximum available current, which is contentious depending on who you ask).
I had to try it for myself, so I ordered some 18650 battery holders from ebay.
My initial conclusion is: SUCCESS. It seems to work just fine. What you see here is not pretty but it works. No doubt future iterations will expect to look more glamorous and more resilient to work in the field.
Yes, there is a major current train during capacitor charging, but it lasts for only about 2 seconds on charged batteries on maximum setting (1/1), while a blink of an eye on minimum (1/128). After a few cycles of maximum discharge you can feel the batteries get slightly warm. On a real shoot one would be swapping out a stale set for a fresh set as needed, and not wait to run the batts into the ground, so I'm sure this will work out just fine, and you can pack a lot more power now.
Here is some voltage readings. The low blip is just after a full power flash went off.
Will update this post later, when there is new information.
In the next iteration removed most of the rubber bands