Assuming a ’123 will keep a more or less steady voltage, you just gut the driver and stick in the appropriate-value resistor.
The little Thorfire KL02 doodads (also made for ’123s ’though 16340s will run hot) do exactly that, “bead” type LED and a resistor on one contact-board, that’s it.
Some dewd was selling them on fleabay 10 for 20bux or something, obscenely cheap. I got 2 duds in a 10pak, but it was still worth it.