Hi JPA, pm me when you get time, nice to see you again. 18650 tube is doable and not too expensive if you got the specs and know the right people. The problem with 18650 is not the tube, but the fact that it is only 1 battery and the circuit board is geared up for up to 8v (?) not 3.7. I ve used a 17670 in my TMax briefly but then it shuts down thinking the battery is too low. 2 18350's came to mind but couldn't find any with enough capacity to warrant having a custom tube made. So I m using 2x rcr123 batteries by a UK company called Exposure (they do bicycle and marine lights. 700mah capacity seems to be the best I ve run in my T Max so far.
I had a brass tube made and currently working on a titanium project. Look on YouTube for sdsteveb