Possibly a BLF 17 zener driver might work for you
https://oshpark.com/shared_projects/d7oYStVQ
Or the BLF17DD zener
https://oshpark.com/shared_projects/bXpxvUdO
Not really single mode, but they should handle stacked 16340’s
They can be found here
They do sell 16650 Li Ion batts