So glad to hear your positive first impression.
There are some 18650 rotaries, none current production.
Nitecore srt7. No experience
Reports of some people legoing convoy tubes in the wtb rrt-01 thread
Search 18650
Here is a lego someone posted there
I tried a sunwayman v20c, it is 18650, rotary
But it wont lego to my rrt-01
Im still looking for a good 18650 tube option
Sunwayman, jetbeam, and nitecore made rotaries, back around 2012