My Transmitter uses 4 AA Batteries (6 volts), so I used a Mini360 (MP2307) 2A Buck Converter Module, soldered the output to 2 empty AA battery terminals, adjusted the output voltage to 6.00 volt, and done!
Now I can connect any 2S up to 6S LiPo battery, without any problem.
You could probably use a single lithium cell to power it as the electronics will run at 3.3v so it will probably be fine down to around 3.5v
The transmitter won’t work when the voltage drops below 4.5 volt.
Also, it drains AA batteries very fast; but with this solution, it last almost forever!